Browsing by Author "Sabnis, Kshitij"

Now showing 1 - 9 of 9
  • Thumbnail Image
    ItemOpen Access
    Characteristics of shock-induced boundary layer separation on nacelles under windmilling diversion conditions
    (AIAA, 2023-06-08) Boscagli, Boscagli; MacManus, David G.; Tejero, Fernando; Sabnis, Kshitij; Babinsky, Holger; Sheaf, Christopher
    The boundary layer on the external cowl of an aero-engine nacelle under windmilling diversion conditions is subjected to a notable adverse pressure gradient due to the interaction with a near-normal shock wave. Within the context of Computational Fluid Dynamics (CFD) methods, the correct representation of the characteristics of the boundary layer is a major challenge to capture the onset of the separation. This is important for the aerodynamic design of the nacelle as it may assist in the characterization of candidate designs. This work uses experimental data obtained from a quasi-2D rig configuration to provide an assessment of the CFD methods typically used within an industrial context. A range of operating conditions is investigated to assess the sensitivity of the boundary layer to changes in inlet Mach number and mass flow through a notional windmilling engine. Fully turbulent and transitional boundary layer computations are used to determine the characteristics of the boundary layer and the interaction with the shock on the nacelle cowl. The correlation between the onset of shock induced boundary layer separation and pre-shock Mach number is assessed and the boundary layer integral characteristics ahead of the shock and the post-shock recovery evaluated and quantified. Overall, it was found that the CFD is able to discern the onset of boundary layer separation for a nacelle under windmilling conditions.
  • Thumbnail Image
    ItemOpen Access
    Characteristics of shock-induced boundary-layer separation on nacelles under windmilling diversion conditions
    (AIAA, 2023-11-02) Boscagli, Luca; MacManus, David; Tejero, Fernando; Sabnis, Kshitij; Babinsky, H.; Sheaf, Christopher
    The boundary layer on the external cowl of an aeroengine nacelle under windmilling diversion conditions is subjected to a notable adverse pressure gradient due to the interaction with a near-normal shock wave. Within the context of computational fluid dynamics (CFD) methods, the correct representation of the characteristics of the boundary layer is a major challenge in capturing the onset of the separation. This is important for the aerodynamic design of the nacelle, as it may assist in the characterization of candidate designs. This work uses experimental data obtained from a quasi-2D rig configuration to provide an assessment of the CFD methods typically used within an industrial context. A range of operating conditions are investigated to assess the sensitivity of the boundary layer to changes in inlet Mach number and mass flow through a notional windmilling engine. Fully turbulent and transitional boundary-layer computations are used to determine the characteristics of the boundary layer and the interaction with the shock on the nacelle cowl. The correlation between the onset of shock-induced boundary-layer separation and the preshock Mach number is assessed, and it was found that the CFD is able to discern the onset of boundary-layer separation.
  • Thumbnail Image
    ItemOpen Access
    Design of a new test rig to investigate transonic external fan cowl separation
    (Association Aeronautique Astronautique de France, 2022-03-28) Sabnis, Kshitij; Boscagli, Luca; Swarthout, Avery; Babinsky, Holger; MacManus, David; Sheaf, Christopher T.
    Ultra high-bypass ratio engines, which show considerable promise in reducing the environmental impact of commercial aviation, generally adopt slim fan cowl profiles. These geometries can be more sensitive to separation on the external surfaces in engine windmilling conditions during take-off climb out or during cruise. This paper describes the development of a two-dimensional wind tunnel rig which can accurately replicate the separation mechanisms experienced by real aero-engine nacelles. This design process highlights the importance of considering factors such as Reynolds-number effects, tunnel-wall effects, the two-dimensional nature of the rig, and the tunnel boundary layers.
  • Thumbnail Image
    ItemOpen Access
    Design of a quasi-2D rig configuration to assess nacelle aerodynamics under windmilling conditions
    (AIAA, 2023-06-08) Boscagli, Luca; Tejero, Fernando; Swarthout, Avery; MacManus, David G.; Sabnis, Kshitij; Babinsky, Holger; Sheaf, Christopher
    Aero-engine nacelles are typically designed to fulfil both design and off-design aircraft manoeuvres. Under-off design conditions one of the objective is to avoid large flow separation either on the external cowl or within the intake that can influence aircraft and engine operability. One particular scenario is represented by a low engine mass flow regime associated with one inoperative engine, also known as a windmilling condition. Under windmilling, the boundary layer on the external cowl of the nacelle can separate either due to the interaction with shockwaves or due to notable adverse pressure gradient towards the trailing edge. Both mechanisms are computationally difficult to model and there is a need for more validation of computational fluid dynamics (CFD) methods. The aim of this work is to develop a rig configuration which will provide CFD validation data for the aerodynamics of a nacelle under representative windmilling conditions. Two flight regimes are considered, namely windmilling diversion and end-of-runway. CFD simulations of a 3D nacelle are used to determine primary aerodynamic mechanisms associated with boundary layer separation. Two rig configurations are developed and both 2D and 3D CFD analyses are used to achieve the design objectives. Overall, this work presents the design philosophy and methods that were pursued to develop a quasi-2D rig configuration representative of the aerodynamics of 3D-annular aero-engine nacelles under windmilling conditions.
  • Thumbnail Image
    ItemOpen Access
    Experimental investigation of external fan cowl separation for compact nacelles in windmilling scenarios
    (AIAA, 2023-06-08) Sabnis, Kshitij; Boscagl, Luca; Babinsky, Holger; MacManus, David G.; Sheaf, Christopher T.
    The slim fan cowl profiles used for ultra-high bypass ratio aircraft engines are designed considering off-design operating conditions, such as engine windmilling during take-off climb out or during cruise. The current paper describes wind tunnel experiments studying how incoming Mach number and engine mass-flow rate influence the aerodynamics governing external fan cowl flow separation in both these windmilling scenarios. A transonic region may form on the forebody surface if the engine becomes inoperative during take-off climb out, with peak Mach number up to 1.2. The subsequent adverse pressure gradient can separate the local boundary layer, resulting in flow separation which originates near the highlight and a more uniform fan cowl pressure distribution. Meanwhile, engine shut down during cruise results in a large supersonic region on the external fan cowl surface which terminates in a normal shock wave. When the Mach number of this shock exceeds about 1.35, a closed separation bubble develops, which causes up to a four-fold increase in the boundary-layer thickness downstream of the shock wave.
  • Thumbnail Image
    ItemOpen Access
    Experimental investigation of transonic external fan cowl separation
    (Association Aeronautique Astronautique de France, 2023-03-29) Sabnis, Kshitij; Boscagli, Luca; Babinsky, Holger; MacManus, David; Sheaf, Christopher T.
    When a civil aircraft engine is shut down during the cruise phase of flight and thus begins to windmill, a supersonic region forms on the external surface of the fan cowl. The terminating normal shock can separate the turbulent boundary layer developing on this external surface. A series of experiments are performed in a quasitwo-dimensional wind tunnel rig to investigate the influence of various parameters on this flow problem. As the engine mass-flow rate is reduced, an increase in normal shock strength results in the onset of flow separation which thickens the boundary layer developing on the external fan cowl surface by a factor of three. A reduction in incoming Mach number from the nominal value of 0.65 to 0.60 weakens the shock wave and thus delays flow separation. If the incoming boundary layer is laminar rather than turbulent, the normal shock Mach number is observed to increased by 10%. Despite the stronger shock, no significant flow separation can be detected even for the lowest engine mass-flow rates studied and the external nacelle surface boundary layer is measured to be thinner than for the turbulent case.
  • Thumbnail Image
    ItemOpen Access
    Numerical and experimental investigations of diffusion-induced boundary layer separation on aero-engine nacelles
    (Elsevier, 2024-10) Boscagli, Luca; Sabnis, Kshitij; MacManus, David G.; Babinsky, Holger; Tejero, Fernando; Sheaf, Christopher
    Aero-engine nacelles have to fulfill design requirements at both cruise and off-design conditions. Under engine windmilling conditions the ingested streamtube massflow is relatively low. A key off-design condition is take-off, which, in conjunction with an engine windmilling scenario, results in the stagnation point of the ingested streamtube being located significantly inside the intake. The combination of high angle of attack and low engine massflow rates leads to a strong flow acceleration and subsequent diffusion of the boundary layer on the upper quadrants of the external nacelle cowl, which can terminate with subsonic separation from the leading-edge. Under this condition, Reynolds number effects can play a dominant role on the separation onset and characteristics and 3D-annular wind tunnel tests cannot always achieve Reynolds’ number equivalent to full scale. A novel quasi-2D rig configuration representative of the aerodynamics of a full-size aero-engine nacelle under windmilling end of runway conditions examined in detail the characteristics of the boundary layer on the external cowl of a nacelle prior to diffusion-induced separation. Separation of the boundary layer was independently promoted through changes to represent different engine massflow rates and freestream Mach number on the rig to determine the limits of steady Reynolds Averaged Navier Stokes (RANS) methods to discern the onset of boundary layer separation. For the conditions and geometry investigated, the combined experimental and computational results showed that there was laminar to turbulent transition of the boundary layer ahead of the subsonic diffusion. The work showed that steady RANS can predict the onset of boundary layer separation with an uncertainty of approximately 10% on notional engine massflow rate and 0.05 on freestream Mach number relative to a nominal operating freestream Mach number of 0.25. This provides guidance for the industrial design and optimization of future windmilling-tolerant nacelles for large ultra-high bypass ratio turbofan engines.
  • Thumbnail Image
    ItemOpen Access
    Shock-induced fan cowl separation during aeroengine windmilling at diversion from cruise
    (American Institute of Aeronautics and Astronautics (AIAA), 2024-11-13) Sabnis, Kshitij; Babinsky, Holger; Boscagli, Luca; MacManus, David; Sheaf, Christopher
    When a civil aircraft engine is operated at windmill during the cruise flight phase, there is supersonic flow acceleration around the leading edge of the fan cowl toward the external surface. The terminating normal shock wave can separate the turbulent boundary layer developing on this external surface. A series of experiments at a flight-relevant Reynolds number (1.2 million based on lip thickness) are performed in a quasi-two-dimensional wind tunnel rig to investigate the underlying flow physics. At a nominal inflow Mach number of 0.65 and a nacelle incidence angle of 4.5 deg, as the equivalent engine mass-flow rate is reduced, an increase in shock strength results in flow separation when the shock exceeds Mach 1.4. Over a 10% range in the notional engine mass-flow rate, the boundary layer developing on the external fan cowl thickens by a factor of three on the onset of separation. A reduction in the incoming Mach number from 0.65 to 0.60 weakens the shock wave and thus delays separation. An increase in surface roughness has no significant effect in situations where the boundary layer remains attached. However, for separated cases, an increased local roughness height causes a greater separation extent and a thicker boundary layer downstream of the shock wave.
  • Thumbnail Image
    ItemOpen Access
    A wind tunnel rig to study the external fan cowl separation experienced by compact nacelles in windmilling scenarios
    (AIAA, 2023-01-19) Sabnis, Kshitij; Babinsky, Holger; Boscagli, Luca; Swarthout, Avery; Tejero, Fernando; MacManus, David; Sheaf, Christopher
    Ultra high-bypass ratio aircraft engines adopt slim fan cowl profiles which are sensitive to separation on their external surfaces under diversion windmilling conditions, when engine shutdown occurs during cruise. A two-dimensional wind tunnel rig is developed to investigate the separation mechanisms experienced by aero-engine nacelles in such scenarios to establish the detailed aerodynamics. The tunnel flow field with entry Mach number 0.65 broadly replicates the expected diversion windmilling flow behaviour for a full-scale nacelle, featuring a supersonic region which terminates in a normal shock wave on the external fan cowl surface. For the three conditions investigated, which relate to different engine mass-flow rates, the response of the boundary layer in this region progresses from remaining fully attached through to well-established separation. However, the rig Reynolds number based on nacelle lip thickness is about 1.2 million, roughly one-third of full scale, resulting in a transitional rather than turbulent shock-boundary-layer interaction. Nevertheless, simple simulations correctly predict the wind tunnel flow field, except for the precise boundary-layer response to normal shock waves, and so experimental data from the rig can be used to validate relevant computational methods.