Browsing by Author "Coskun, Seyfettin"

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    Acoustic excitation as a flow control technique in a high-speed compressor cascade
    (American Society of Mechanical Engineers, 2024-06-24) Coskun, Seyfettin; Rajendran, David John; Pachidis, Vassilios; Bacic, Marko
    A numerical investigation of the effectiveness of acoustic excitation is carried out to control flow separation in a NACA65-K48 linear compressor cascade (LCC) in the absence of cascade endwalls. The operating conditions for the LCC are Ma = 0.67 and Rec = 560 × 10^3, which are representative of an aeroengine. For the numerical investigations, Improved Delayed Detached Eddy Simulation (IDDES) is used due to its capability to capture flow separation and unsteady flow characteristics with reasonable computational requirements among the high fidelity approaches. The flow through the cascade passage does not experience any separation at the aerodynamic design point (ADP) of the cascade. Therefore, an incidence angle of i = 8° is used in this study where flow separation is observed over blade the suction surface at x/c ≈ 0.6. For acoustic excitation, both external and internal acoustic excitation techniques are investigated. In external acoustic excitation, the sound source is distributed at the inlet boundary of the computational domain whereas in internal excitation, sound waves are introduced into the computational domain from a thin slot located at the onset of flow separation on the blade suction surface. Sound waves are introduced in the flow field at the modal frequencies in the uncontrolled flow, which are Stc = 0.187, 0.409, and 0.618 in terms of Strouhal number based on the blade chord length at a constant excitation amplitude of SPL = 151dB. The results have indicated that external acoustic excitation has no major effect in controlling flow separation in the LCC. Internal acoustic excitation, on the other hand, has a strong effect in modulating the flow separation in the LCC passage such that a reduction of Δζmax ≈ 16% is observed in the total pressure loss coefficient on the measurement plane located 0.4Cax downstream of the cascade exit. Further investigations are also carried out for the effect of sound amplitude on the effectiveness of internal acoustic excitation. In overall, acoustic excitation can serve as a means of flow control in aeroengine compressor cascades.
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    Control of flow separation in a high-speed compressor cascade through acoustic excitation
    (AIAA, 2023-06-08) Coskun, Seyfettin; Rajendran, David John; Pachidis, Vassilios; Bacic, Marko
    The use of acoustic excitation for controlling flow separation in a NACA65-K48 linear compressor cascade operating at aircraft engine representative Ma = 0.67 and chord-based Re = 560,000 is investigated numerically. Improved Delayed Detached Eddy Simulation (IDDES) is used for numerical simulations. The linear compressor cascade passage under investigation is subject to severe secondary flows that are the fundamental loss mechanisms in axial compressors. Secondary flows such as corner separation cover a significant portion of the blade height of the linear compressor cascade (LCC) because of its low aspect ratio. These losses result in passage blockage which results in performance degradation. In the current study, the effect of external acoustic excitation on flow separation characteristics in the compressor passage is examined. The effectiveness of acoustic excitation is investigated for two main excitation parameters: excitation frequency and amplitude. The dominant frequencies in the uncontrolled flow frequency spectra are used as the initial excitation frequencies whilst a range of excitation amplitudes are considered. It has been observed that when the acoustic excitation is applied with a frequency in the range of the most dominant frequency in the uncontrolled flow and an excitation amplitude above a threshold amplitude, the flow field can be modulated substantially to recover the cascade performance.
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    Control of flow separation over an aerofoil by external acoustic excitation at a high Reynolds number
    (AIP Publishing, 2024-01-05) Coskun, Seyfettin; Rajendran, David John; Pachidis, Vassilios; Bacic, Marko
    The effectiveness of acoustic excitation as a means of flow control at high Reynolds number turbulent flows is investigated numerically by using Improved Delayed Detached Eddy Simulations. Previous studies on low Reynolds number laminar flows have shown that acoustic excitation can substantially suppress flow separation for specific effective frequency and amplitude ranges. However, the effect of acoustic excitation on higher Reynolds number turbulent flow separation has not yet been explored due to limitations on appropriate fidelity computational methods or experimental facility constraints. Therefore, this paper addresses this research gap. A NACA (National Advisory Committee for Aeronautics) 0015 aerofoil profile at 1 million Reynolds number based on the aerofoil chord length is used for the investigations. Acoustic excitation is applied to the baseline flow field in the form of transient boundary conditions at the computational domain inlet. A parametric study revealed that the effective sound frequency range shows a Gaussian distribution around the frequency of the dominant disturbances in the baseline flow. A maximum of ∼ 43% increase in lift-to-drag ratio is observed for the most effective excitation frequency F+ = 1.0 at a constant excitation amplitude of Am = 1.8%. The effect of excitation amplitude follows an asymptotic trend with a maximum effective excitation amplitude above which the gains are not significant. A fully reattached flow is observed for the highest excitation level considered (Am = 10%), that results in ∼ 120% rise in aerofoil lift-to-drag coefficient. Overall, the findings of the current work demonstrate the higher Reynolds number effectiveness of acoustic excitation on separated turbulent flows, thereby paving the way for application in realistic flow scenarios observed in aircraft and gas turbine engine flow fields.
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    Improved delayed detached eddy simulations applied to a NACA0015 aerofoil subject to acoustic excitation
    (AIAA, 2022-06-20) Coskun, Seyfettin; Pachidis, Vassilios; Ubulom, Iroizan; Bacic, Marko
    The effect of acoustic excitation on the aerodynamic performance of NACA0015 aerofoil at $\alpha=11^{\circ}$ and $Re_c = 1.0 \ x 10^6$ based on $c=0.35m$ chord length is investigated numerically using Improved Delayed Detached Eddy Simulations (IDDES). Flow separation is observed over the suction surface in the uncontrolled flow that forms a separated shear layer, resulting in aerodynamic performance degradation. External acoustic excitation is used in controlling the flow separation by exciting the separated shear layer. Acoustic excitation frequency and amplitude are the control parameters considered. The flow field is excited at a frequency of the most amplified disturbances and at first harmonic and sub-harmonic frequencies with a constant excitation amplitude. Unlike the harmonics, when the separated shear layer is excited at $F+=1$, the separation characteristics of the uncontrolled flow field are significantly altered. Unsteady disturbances are locked into the excitation frequency and transition is promoted within the shear layer. A separation bubble is generated over the suction surface with a reattachment region aft of the aerofoil surface. Vortex coherence and organized wake structures are also improved. The effect of excitation amplitude is investigated at the frequency of the most amplified disturbances. Increasing the excitation amplitude has two predominant impacts; it delays the boundary layer separation and advances the reattachment location, thereby reducing the separation bubble length. The results also suggest that there is a maximum effective acoustic excitation amplitude.