This paper presents the aeroacoustic analysis of a lift-offset coaxial rotor in high-speed forward flight using the high-fidelity computational fluid dynamics/computational structural dynamics (CFD/CSD) loose coupling software Helios. Acoustic simulations are performed using the software PSU-WOPWOP at eight microphones positioned below the coaxial rotor. The total power of the three speed cases—100, 150, and 200 kt—is validated against flight-test data and shows good agreement. A series of parametric studies is also conducted to investigate the effect of lift offset, flight speed, and rotor-to-rotor separation distance on acoustics of the coaxial rotor. Strong blade-crossover and self-blade–vortex interaction events of the coaxial rotor, which are major sources of loading noise, are captured via high-fidelity CFD simulations in all speed cases. Highly impulsive acoustic pressure signals are identified in all simulation cases, and the magnitude of mid-frequency sound pressure level (SPL) increases significantly with increasing flight speed and lift offset. The strength of mid-frequency SPL, on the other hand, is reduced significantly as the rotor-to-rotor separation distance increases at 100 kt. However, the higher speed cases do not show a significant reduction in mid-frequency SPL with increasing separation distance.
Currently, 3D Computational Fluid Dynamic (CFD) rotorcraft simulations are able to account for blade crossover interaction (BCI) events, which are impulsive loading events that have a large influence on the vibrations and acoustics of a vehicle. Unfortunately, lower fidelity models are unable to adequately predict the BCI events and 3D CFD simulations are computationally expensive. This paper proposes a surrogate model that is able to predict the BCI event with reasonable accuracy and high computational efficiency for a subset of operating conditions. A dataset was created using transient 2D CFD simulations of airfoils moving toward each other in close proximity. The Mach number, angle-of-attack of each airfoil, along with the vertical separation distance between airfoils was varied in each simulation. An additional dependent parametric input (airfoil horizontal separation distance) was recorded along with the transient loads on the airfoils. This dataset was used in a supervised manner to train univariate (UV) and multivariate (MV) implementations of the Gaussian Process Regression model. Given airfoil operating conditions, the models are able to predict the BCI events. Hyper-parameters such as kernels and trend functions were compared using 5-fold cross validation and the final MV and UV models were compared on a held out test set to demonstrate predictive performance on unseen data.
Current paper summarizes a correlation study of two flow solvers (CREATE-AV™ Helios and STAR-CCM+), routinely used at Sikorsky, with the spinning coaxial hub drag and flow field measurements conducted by Penn State University at the 12' diameter water tunnel. The Helios modeling approach was aiming for a high fidelity accurate simulation, whereas the STAR-CCM+ modeling approach was aiming for a fast turn-around time with reasonable solution accuracy with a relatively coarse mesh and simplification. The two solvers generally agreed well with the test data within reasonable accuracy and captured the drag trend between two shaft fairing configurations. Impact of turbulence model selection (Spalart-Allmaras Detached Eddy Simulation and Spalart-Allmaras Reynolds-Averaged-Navier-Stokes model) has been demonstrated. The RANS model generally delayed separation and resulted in lower drag. The STAR-CCM+ runs simulated both air and water at matching Reynolds number and showed good agreement between the drag results for the two mediums. Also, the importance of accurate representation of geometric details including gaps, shafts, and holes is highlighted.
This paper performs aerodynamic and acoustic analysis on a coaxial rotor system based on the XH-59A coaxial helicopter. The aerodynamic analysis is performed in RCAS using free wake modeling, and the acoustic prediction is performed with PSU-WOPWOP, which utilizes Farassat's Formulation 1A of the Ffowcs Williams-Hawkings equation. The coaxial rotor system is analyzed for forward flight speeds of 100 knots and 150 knots. For each case the rotor shaft angle and the distribution of loading is varied to understand the impact on the rotor noise and the aerodynamic environment of the rotor system. The aerodynamic environment is analyzed by wake and blade load visualization, and the noise is computed on a hemi-spherical grid 10 rotor radii from the coaxial rotor system.
This paper investigates the effect of pitch attitude on both performance and acoustics of a lift-offset coaxial rotor based on a first-principles and high-fidelity CFD/CSD loose coupling approach at 150 and 200 knots. The pitch attitudes selected for this research are -5°, 0°, and 5°. The CFD/CSD loose coupling simulations are carried out using the CREATE™-AV software Helios while the coaxial rotor acoustics is simulated using PSU-WOPWOP at eight microphones positioned below the lower rotor. A detailed aerodynamic analysis is performed at 150 knots. A total of six major aerodynamic interactions are identified: 1) hub-wake interaction,2) self-BVI, 3) parallel rotor-to-rotor BVI, 4) blade-crossover events, 5) root-induced BVI, and 6) reversed-flow edge-vortex interactions. The strength of these interactions is dependent on the vehicle pitch attitude. The cases with negative pitch attitude show significantly stronger impulsive pressure pulses, which is found to be induced by parallel rotor-to-rotor BVIs of the lower rotor. Moreover, the positive and zero pitch attitude cases tend to dominate the acoustic region on the starboard side of the coaxial rotor, whereas the negative pitch attitude case shows higher acoustic pressure peaks on the port side. Overall, the case with a positive pitch attitude shows significant improvement in rotor aerodynamic efficiency, rotor acoustics, and vehicle power performance at high speed. The hemispherical acoustic simulation results at 150 knots also show that the noise is less likely to propagate in the forward direction with a positive pitch attitude.
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