Browse Topic: Turbulence
The performance and acoustics of a scaled propeller designed for an eVTOL vehicle were investigated in axial and edgewise flight. The measured performance compared well with BEMT predictions in axial flight conditions. The noise produced by the propeller is dominated by broadband noise sources, where there is evidence of contributions from blade wake interaction noise, turbulent boundary layer trailing edge noise, and laminar boundary layer vortex shedding noise. The directivity of the noise was found to be dependent on the advance ratio. Beamform maps also identified changes in the dominant noise source at different observer locations as a function of advance ratio.
This study presents computational analyses of coaxial rotor hub flows and validation against experimental data obtained from the fifth Rotor Hub Flow Prediction Workshop. Experiments were conducted in a 12-inch diameter water tunnel at Pennsylvania State Applied Research Laboratory, employing tomographic particle-image velocimetry (Tomo-PIV) and precise hub drag measurements. Three CFD codes (UMD Mercury, CREATETM-AV Helios, and OVERFLOW) utilizing hybrid Reynolds-Averaged Navier-Stokes (RANS) / Large Eddy Simulation (LES) modeling based on Spalart–Allmaras turbulence model, were applied to replicate and analyze hub flows. Counter-rotating coaxial rotor hubs under free-air condition was simulated as the simplest case and the hub drags are compared between the three CFD codes. The full water tunnel configuration, consisting of two hubs, a fairing, and shafts, was also simulated and compared to experimental results, with a focus on hub drag, wake velocity fields, and turbulence
This paper expands on a previous exploratory investigation into the safety implications of helicopter operations at hospital landing sites. The paper analyses the interaction between rotor downwash, the turbulent wake shed from nearby buildings and the effect of varying windspeed and aircraft position. A RANS CFD method has been used to compute the mean airflow in the vicinity of a hospital helipad with a helicopter, representative of a Bell 412, hovering at three different positions around the site. The main rotor of the aircraft was modelled using a Virtual Blade Model, enabling a coupled solution between the airflow around nearby structures and the helicopter. The study examines the resulting airflow patterns and velocity magnitudes around the site for two incoming windspeeds and three varying aircraft positions. Results presented are focussed on areas where the rotor downwash is present and likely to impact pedestrians. The findings show that windspeed can affect how the downwash
This study introduces a structured methodology for identifying Control-Equivalent Turbulence Input (CETI) models using rotorcraft flight dynamics simulations. A new Moving Spatial Turbulence Field (MSTF) model was developed to generate input datasets, enabling CETI model identification for four distinct aircraft configurations: a generic utility helicopter resembling the H-60, and three small-scale multi-rotor UAS types—a quadcopter, hexacopter, and octocopter. The CETI models were validated in hover using frequency-domain analysis, with flight-derived CETI models serving as the benchmark. To further assess model performance in forward flight, CETI models for the H-60 were identified at airspeeds ranging from 0 to 140 knots in 40- knot increments. Results indicated that the MSTF-based CETI models for the H-60 effectively captured key spectral features of the flight-test data, though some deviations were observed, potentially due to variability in atmospheric conditions. In contrast
Precision flight in windy conditions is a common challenge for multirotor UAS. It is especially challenging for in contact tasks that require high-precision positioning and good disturbance rejection capabilities. Such tasks include landing on high-voltage powerlines for in-contact inspections. This paper presents the implementation of small lateral thrusters to improve the lateral position hold ability of a large power line inspection UAS in windy conditions. Arranged in antagonistic pairs on each side, the lateral thrusters handle the high-frequency but smaller-amplitude wind turbulence components with a frequency split control. Using an identified model of the UAS flight dynamics alongside flight data in high-wind conditions, a control architecture with a frequency split in the lateral axis was optimized to increase the disturbance rejection. Experimental tests showed a 67% reduction in lateral position error with the proposed approach in high-wind conditions.
This study presents the development and application of a refined momentum source term methodology for synthetic turbulence generation in urban flow simulations. By embedding divergence-free, three-dimensional turbulence fields consistent with the von Kármán energy spectrum directly within the computational domain, the approach enables flexible and efficient turbulence generation with minimal sensitivity to grid stretching. The method is validated through Large Eddy Simulations (LES) of flow around a representative urban vertiport model under varying turbulence intensities (10%, 20%, and 30%). Results demonstrate that the generated synthetic turbulence significantly alters the flow field, reducing recirculation zones, promoting earlier shear-layer reattachment, and stabilizing the flow above the vertiport platform—key factors for safe eVTOL operations. Instantaneous flow analyses reveal that secondary tip vortices (STVs) persist even in the presence of strong inflow turbulence but lose
An extensive test campaign was conducted at the National Full-Scale Aerodynamics Complex 40- by- 80-Foot wind tunnel to acquire performance, loads, and acoustics measurements of the Joby Aviation propeller across a variety of operating conditions. The dataset provided validation of the design methodology as well as verification of computational tools. The Vold-Kalman filter was used to extract the shaft-coherent propeller noise in hover to obtain the residual noise, representing the broadband noise. This data verified broadband noise tip speed scaling laws as well as a low-order empirical model for overall sound pressure level. The OVERFLOW/PSU-WOPWOP method was used to simulate the propeller in pure edgewise flight and shown to accurately predict propeller performance. The low-frequency acoustics were predicted well but the solver underpredicted frequencies above 300 Hz, possibly due to the inability to capture the turbulent component of the blade-wake and blade-vortex interaction
This study presents a statistical approach for detecting and estimating damage to multicopter propellers through a comprehensive probabilistic model. The methodology is derived from model-based analysis and applied within the time series statistical techniques. This research accounts for uncertainties in the estimation process and offers confidence intervals for assessing the extent of damage to the propellers. The framework employs functionally pooled (FP) models characterized by parameters that depend on damage sizes, proper statistical estimation, and decision-making schemes. The validation and assessment are assessed via a hexacopter flying in circles with a constant velocity and altitude under turbulence. The damage size ranges from healthy to 10 mm. The method achieves fast damage detection and precise magnitude estimation based on a segment of a single measured signal obtained from aircraft sensors during flight.
A computational study is conducted on a coaxial rotor hub and sail fairing configuration to analyze hub surface forces and the characteristics of its downstream wake. The flow conditions and grids are based on experimental tests performed at the Penn State Applied Research Lab (ARL) Water Tunnel at a baseline Reynolds number. Grid development for the rotor hubs and sail fairing is done using Pointwise v18.04R1 and Chimera Grid Tools (version 2.2). Simulations are performed using NASA's OVERFLOW2.4b Reynolds Averaged Navier-Stokes solver. The drag forces on the rotor hubs are computed and compared to standalone drag data to analyze the effects of interactional aerodynamics. Flow features, frequency content and Reynolds stresses of the wake are analyzed. Frequency content and Reynolds stresses show clear spatial bias. The anisotropy of the Reynolds stresses is computed and used to determine the character of the wake turbulence.
This study models flow around isolated and side-by-side three-bladed propellers in (IGE) and out of ground effect (OGE) using actuator-based techniques of varying fidelity. Actuator techniques model propellers using momentum sources distributed over the disk in actuator disk method (ADM) or distributed over moving lines in actuator line method (ALM) to reduce computational cost compared to blade-resolved DDES simulations. The lowest fidelity ADM method is observed to reasonably predict thrust with the use of a tip loss model to control runaway thrust at the tip while not resolving flow features such as blade-bound vortices and helical tip vortices at a fraction of the cost of BR-DDES (1/100). The coarser ALM model resolves these features but still requires a tip loss model to control runaway thrust at 1/10th the cost of BR-DDES. Finally, the finer ALM model used in this study accurately captures blade-related features and further predicts the tip loss trend from first principles at 1
The development of turbulence criteria to provide early guidance for the design of vertiports is presented in this paper. For any aircraft, winds, in particular crosswinds and gusty winds, are top of mind for all pilots engaging in take-off and landing maneuvers. It is anticipated that the same will be true for VTOL and eVTOLs landing on vertiports, in particular as new vertiports are built closer and closer to urban centres. First, a review of the current design criteria for vertiports around the world related to wind is presented, highlighting the commonality between the guidance and the gaps in their content. Second, the controllability criteria that VTOL and eVTOLs will likely need to meet in the pursuit of an airworthiness certification are reviewed and their pertinence with regards to vertiport design are discussed. Third, the characters of the wind and their impact on eVTOL flights at or near take-off and landing infrastructure is explored. Finally, a set of turbulence criteria
A use-case was conducted in Montréal in the summer and fall of 2023 to measure urban airflow characteristics using a small Remotely-Piloted Air System (sRPAS). The goal of the study was to acquire urban airflow data in a real environment in order to validate urban airflow characteristics from laboratory-scale testing conducted previously. The use-case took place in the downtown core of Montréal and involved flights from two hospitals to a variety of other buildings. The sRPAS was instrumented with an airflow measurement system. Fixed rooftop anemometer stations were also installed on top of buildings along the flight paths to measure urban airflow at altitudes within close proximity to rooftops. The study generated a valuable data set for characterizing sRPAS operations in urban environments. A number of operational challenges were experienced including the difficulty associated with visual line of sight operations with an urban backdrop, avoiding conditions that could lead to loss of
ABSTRACT A proof of concept test to measure the unsteady boundary layer transition locations on the lower surface of a Machscaled rotor in forward flight was performed during the Summer of 2017 in the NASA Langley 14- by 22-Foot Subsonic Tunnel. The transition locations were measured using high-speed infrared thermography with a rotating mirror assembly that could be remotely actuated to acquire data at several rotor azimuths. Data were acquired for eight unique rotor flight conditions for a range of advance ratios (μ=0:10 : 0:38), thrust coefficients (CT/α =0:04 : 0:12) and rotor shaft angles (αs = -6 deg : 0 deg). This paper presents the transition locations as a function of azimuth and radius for an advance ratio of, μ, of 0.30, and thrust coefficent, CT/α, of 0.08. At this condition, the lower surface is fully laminar on the retreating side and mostly turbulent on the advancing side except near the tip. The tip airfoils were greater than 60 percent laminar on the lower surface
The Shake-The-Box technique was applied to experimentally quantify the time-resolved volumetric flow field around a free-flying quadcopter UAV with an overall span of about 0.5 m. State-of-the-art LED illumination and high-speed camera equipment was combined with modern Lagrangian tracer particle tracking and data assimilation techniques, facilitating a measurement volume larger than 1.5m3. The setup allowed for both hover and limited maneuvering of the quadcopter, while resolving even small details of the complex interactional aerodynamics. In hover out of ground effect, the four individual rotor wakes merged into a single jet within a few rotor radii below the rotor planes. Evaluating the mass and momentum fluxes over suitable control volumes yields accurate estimates for the quadcopter's total thrust, the asymmetric thrust distribution between front and back rotors, and the entrainment of external flow through turbulent mixing. Hover in ground effect decreases the power requirement
Rotorcrafts frequently operate in environments with severe atmospheric turbulence, for instance transferring people offshore to and from oil rigs as well as operating from and around ships. The presence of high turbulence can deteriorate performance, stability, and controllability of the rotorcraft. Additionally, such challenging conditions also generate loads that both airframe and rotor components must withstand. Following this, it is crucial to consider the impact of these operational atmospheric conditions during rotorcrafts design and development. In this context, numerical models are a fundamental tool to provide an easier and quicker way to explore the operative envelopes of the helicopter compared to performing experimental activities. This paper presents a rotor loads correlation activity between an experimental test designed and carried out by Leonardo Helicopters in which an AW189 helicopter was placed in the wake of a C-27J Spartan aircraft and a multibody structural model
This paper describes wind tunnel testing of small remotely piloted aircraft systems (RPAS) to understand better the maximum wind speeds in which they can be safely operated. Urban flow fields can contain complex flow structures such as speed changes, direction changes, shear layers, turbulence and vorticity; all of these can impact the safety of urban RPAS operations. The work described in this paper is part of an ongoing effort to provide Canadian regulators with knowledge to guide safe RPAS operations in urban environments. In the wind tunnel, flow fields representative of urban flows were created using simple flow manipulators like bluff bodies and vanes. The flow manipulators and the resulting flow fields, in relation to representative urban flows, are described in this paper. Wind tunnel testing of a number of RPAS in these representative airflows was conducted to evaluate the sustained wind speed limit at which the vehicle could maintain a stable hover. These tests enabled a step
The capabilities of two different laminar-turbulent transition models are evaluated for the prediction of the PSP rotor performance in hover. The first transition model originates on non-local semi-empirical transition criteria that are calculated on the basis of the history of boundary layer quantities along the wall streamlines. The second one is the Langtry-Menter model that consists in two additional transport equations based on a local transition criterion. The same numerical methods and same post-processing are used with the elsA CFD solver in order to have a fair comparison between the models. Both transition modeling technics provide a good agreement with the experimental measurements concerning the transition position on the upper side of the blade. On the lower side, the predictions are less satisfactory. Transition criteria approach gives good trends while Langtry-Menter results seem to be polluted by the tip vortex flow. A grid sensitivity study shows that Langtry-Menter
The National Research Council of Canada (NRC) has recently developed an Integrated Reality In-flight Simulator (IRIS) that allows helicopter pilots to fly the NRC's Bell 412 Advanced Systems Research Aircraft (ASRA) while wearing a commercial off-the-shelf (COTS) virtual reality headset. IRIS is the first airborne simulator of its kind that combines COTS virtual reality and Fly-By-Wire (FBW) synthetic turbulence for helicopter operations. Simulations are not exact replications of actual environments; therefore, a methodology of comparing pilot workload with respect to an analysis of the differences between the simulated and actual environments is required. During a recent flight trial, NRC validated the effectiveness of IRIS to replicate a pilot's workload during ship landing tasks using these workload scales. During the analysis, NRC took initial steps in developing methodologies to examine environmental characteristics and then correlate them to an associated pilot workload. The work
Piloted simulation has been used for decades to support flight test activities at the Naval Air Warfare Center Aircraft Division located at Naval Air Station Patuxent River, MD. Conventional lab stations at the Manned Flight Simulator facility have been used effectively to support a wide range of flight test requirements. However, there were limitations with these conventional lab stations when the purpose was to assess handling qualities and pilot workload while landing rotorcraft aboard a ship. Two critical simulation elements were determined to be necessary: (1) an expanded field of view so the pilot could see the ship deck below the aircraft and (2) a motion system to provide the pilot with vital proprioceptive cueing in the turbulent ship environment. A new Virtual Reality Lab was developed at Patuxent River that included these key features. The primary components of the lab included virtual reality headsets, an Unreal Engine image generator, ocean and ship visual models, a six
This paper presents results from an ongoing research that aims to create Parametric Rotor Control Equivalent Turbulence Inputs (RCETI) models for different rotor configurations. In RCETI modeling, the rotor swash-plate deflections are utilized as inputs to match the turbulence-related spectra of rotor hub-loads in order to achieve the parametrization and generalization of these models. The development of the RCETI model, which aims to produce rotor loads spectra similar to those generated by two-dimensional spectra of turbulence, is conducted using a representative rotor model in FLIGHTLAB®. The effect of rotational sampling of turbulence on the rotor response is analyzed. The hub-fixed sampling, rotational sampling at 0.75R as well as the blade-element sampling of turbulence are considered and compared. Furthermore, parametric analysis is carried out to study the effect of altering rotor parameters on the developed RCETI model and presented in the paper.
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A robust framework for fault detection and identification of rotor degradation in multicopters while effectively rejecting the effects of gusts is introduced. The rotor fault detection and identification methods employed in this study are based on excitation-response signals of the aircraft under ambient turbulence to distinguish between an aircraft response to gusts and rotor faults. A concise overview of the development of statistical time series model for healthy aircraft using the aircraft attitudes as the output and controller commands as the input is presented. This model is utilized to extract quality features for training a simple neural network to perform effective online rotor fault detection and identification in a hexacopter exceptional speed of making a decision and accuracy of fault classification. It is shown that using a statistical time series model assisted neural network employed for online monitoring is capable of rejecting gusts, sensitive to even 20% rotor
This work introduces the use of "global" stochastic models to detect and identify rotor failures in multicopters under different operating conditions, turbulence, and uncertainty. The identification of an extended class of time-series models known as Vector-dependent Functionally Pooled AutoRegressive models, which are characterized by parameters that depend on both forward velocity and gross weight, using scalar or vector aircraft response signals under white noise excitation has been described. A concise overview of the residual based statistical decision making schemes for fault detection and identification of rotor failures is provided. The scalar and vector statistical models, along with residual variance and residual uncorrelatedness methods were validated and their effectiveness was assessed by a proof-of-concept application to aircraft flight for healthy and faulty states under severe turbulence and intermediate operating conditions. The results of this study demonstrate the
A turbulence model based on a Synthetic Eddy Method has been adapted for flight simulation purposes and coupled to two FlightLab helicopter models. The model is based on the generation of a random distribution of turbulence generating Eddies within a control model surrounding the aircraft. Eddies are convected by the flow and regenerated at the inflow as they leave the simulation domain. Adjustment of Reynolds stresses and Eddy shape and sizes should allow adjustment of turbulence intensities and frequency spectra. Compared to other random turbulence models, preserving the location of the Eddies in the control volume ensures automatically that turbulence across different aircraft locations is automatically correlated. Offline and piloted flight simulation has been conducted to test the viability of the concept. Results show that the turbulence model generates upsets in all aircraft axis which result in higher workload requirements for the pilot.
This paper presents an efficient prediction of coaxial rotor broadband noise, particularly trailing-edge noise. The method combines a newly developed iterative coaxial rotor BEMT, a viscous panel method, an empirical wall pressure spectrum, and Amiet's trailing-edge noise model. Aerodynamic data including the induced velocity and angle of attack on each rotor are calculated by the iterative BEMT. Then, turbulent boundary layer flows, such as the boundary layer thickness, skin friction coefficient, pressure gradient, etc., are computed by a viscous panel code, XFOIL. Based on these boundary layer parameters, the wall pressure spectrum near the trailing edge is computed by Lee's semi-empirical model. Finally, trailing-edge noise is predicted by Amiet’s model from the wall pressure spectrum. This method provides fast computations for aerodynamics and acoustics for coaxial rotors. Acoustic predictions can be performed for various design and operating conditions including the effect of
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