Browse Topic: Turbulence
Atmospheric turbulence is a major source of uncertainty for unmanned rotorcraft operating in confined or disturbed environments, where robust trajectory planning requires reliable bounds on vehicle response. High-fidelity turbulence models are typically too computationally demanding for onboard use and difficult to integrate into planning frameworks. This paper presents a Control Equivalent Turbulence Input (CETI)–based approach to characterize turbulence effects on the inner-loop dynamics of a small unmanned helicopter and to derive disturbance-induced state deviation bounds suitable for robust planning. CETI models are identified from manually piloted hover flight tests of the unmanned research helicopter midiARTIS using a linear bare-airframe model and a Kalman filter for disturbance estimation. CETI transfer functions are fitted to averaged power spectral densities of the extracted disturbance inputs. The resulting model is validated by reproducing the identified transfer functions and by comparing open-loop simulation results to flight-test data in both time and frequency domain. Based on simulations with CETI inputs, probabilistic bounds on state deviations are derived and related to measured flight-test responses. The results demonstrate that the proposed CETI workflow provides a compact and computationally efficient turbulence surrogate that captures the dominant effects of atmospheric gusts on rotorcraft dynamics and is well suited for inner-loop performance assessment and as an input to robust model predictive control algorithms.
To examine the unsteady aerodynamic environment surrounding vertiports, this research performs high-fidelity Large-Eddy Simulations of two full-scale square cylinders with aspect ratios of 2 and 1, representing prospective urban and vertiport configurations. To ensure realistic conditions, a volumetric momentum-source term is utilized to generate atmospheric turbulence internally within the computational domain. Numerical results reveal severely disturbed flows, with total turbulence intensities reaching a peak of 38% near the first vertiport structure. While the downstream vertiport benefited from a 40% reduction in maximum turbulence intensity due to the sheltering effect of the upstream building wake, the flow in its vicinity still maintains a powerful content. To evaluate operational safety conditions, multiple flight trajectories were analyzed approaching two distinct end-points located 4m and 11m above the center of each vertiport model from three directions: aligned with the wind, against the wind, and sidewise. Based on the evaluation of transient velocity components across these trajectories, and following the Federal Aviation Administration compliant 8:1 approach slopes, this study proposes a robust operational strategy centered on lateral approaches followed by vertical descents from higher altitudes. This maneuver sequence is designed to avoid the severe and sudden gust-induced disturbances prevalent in the immediate vicinity of the vertiport.
A generalized turbulence model for rotorcraft, analogous to the Dryden or von Kármán models commonly used for fixed-wing aircraft, does not yet exist. The closest available formulations are Control-Equivalent Turbulence Input (CETI) models, which reproduce the portion of the vehicle response attributable to atmospheric disturbances through equivalent control inputs applied at the inceptor or effector level. While the underlying concept is broadly applicable, these models are highly configuration dependent, making their broader generalizability uncertain. This paper adopts a recently developed methodology to extract CETI models directly from simulation and extends it to the identification of State-Equivalent Turbulence Input (STETI) models, which are Dryden-like in form and inject turbulence-equivalent excitations directly into the state dynamics. The approach is applied to six conventional main-tail rotor helicopters spanning trainer, light, intermediate, medium, heavy, and ultra-heavy classes, from the Robinson R-22 to the Mil Mi-26, and to four tiltrotors spanning the current range of tiltrotor weight classes, from the Bell XV-15 to the Bell Boeing V-22. The resulting CETI and STETI models are used to examine cross-vehicle trends, assess the extent to which they generalize across configurations, and evaluate whether they can be parameterized in terms of fundamental rotorcraft properties, including gross weight and rotor radius, as well as derived parameters such as disk loading.
The impact of ship airwake on helicopter operations to rear flight decks has been a topic of much research over the past three decades. While generic ships have been developed over the years to enable analysis tool and knowledge development, actual ships can vary significantly, resulting in different airwake features. The study of variations in ship geometry is important both to understand how differences may impact operations, but also to understand the level of geometrical fidelity that is required on ship models undergoing analysis. In Canada, the newly launched (2018) Harry DeWolf-class Arctic Offshore Patrol Ships (AOPS) have unique features that have been studied for their impact on airwake characteristics. This paper explores different geometrical characteristics from the perspective of their operational impacts and also considering their importance for inclusion in simulation. The paper shows that turbulence level is the parameter most affected by the minor variations that were examined, and helps guide the inclusion or exclusion of details in future ship models.
This study experimentally examines the effect of forced boundary layer (BL) transition on the aerodynamic and aero-acoustic performance of a low Reynolds number rotor in hover. An APC 15×4E two-bladed rotor was tested in three configurations: clean, upper-surface trip (U.S.T.), and combined upper- and lower-surface trip (U.S.T./L.S.T.). Surface oil flow visualization was used to characterize the BL structure. A hover test rig was used to measure the static thrust and torque. Acoustic measurements were conducted in an anechoic chamber, with tonal and broadband noise components separated during post-processing. Results show that surface trips effectively force BL transition, increasing turbulent attachment over the blade. Tripped configurations reduced thrust and increased torque but mitigated Reynolds-number sensitivity. Forced transition reduced the tonal noise for all but one case. For the broadband noise, the forced transition increased the noise in the frequency range where turbulent boundary layer-trailing edge (TBLTE) mechanisms dominate, while decreasing the noise in the frequency range where laminar boundary layer vortex shedding (LBL-VS) occurs.
A Rotor Control Equivalent Turbulence Input (RCETI) model for characterizing vehicle response in urban environments turbulent airwakes is investigated. By extracting transfer functions from the nonlinear, high fidelity UH-60 rotorcraft model implemented within the FLIGHTLAB®framework, vehicle response to vertical turbulence is evaluated and inverse mapping between the rotor hub thrust coefficient and the control input spectrum is determined. Furthermore, the RCETI methodology develops filters that produce time history samples of collective input that produce hub loads that are stochastically similar to those induced by atmospheric air wakes.
An aspect of the ship-helicopter dynamic interface (DI) is the highly unsteady flow environment generated by ship-rotor aerodynamic interactions, which challenges safe launch and recovery operations. To investigate these interactions without the constraints of conventional rotor scaling, a novel airflow-and-blade-frequency (ABF) system was developed, decoupling rotor thrust from blade-passing frequency and enabling independent control of disk loading and periodic excitation. Mean-flow superposition and spectral analyses were used to assess the validity of linear-superposition approaches for DI modeling. While superposition reproduced portions of the interacting mean flow, it failed to capture key features such as superstructure sheltering. Spectral results showed that momentum injection and blade-passing frequency modified the interacting flow through distinct mechanisms. Across all operating conditions, the interacting flow exhibited elevated turbulent kinetic energy at pilot-relevant frequencies over a broader spatial extent than either the isolated airwake or the superposed field, indicating that nonlinear aerodynamic interactions generated flow features that super-positional models did not capture. The persistence of these trends across different ABF operating parameters suggested that correction-based approaches may approximate rotor-feedback effects without requiring fully resolved aerodynamic interactions between the ship and rotor (air)wakes.
The present work develops a computational framework for simulating the two-way coupled ship-helicopter dynamic interface using large-eddy simulation. The Simple Frigate Shape 2 geometry is modeled using the immersed boundary method, and baseline simulations under both uniform inflow and neutral atmospheric boundary layer (ABL) conditions are validated against wind tunnel measurements for two wind-over-deck angles. Rotor modeling techniques, including the actuator line model (ALM) and actuator disk model (ADM), are verified and validated across several configurations: the Knight and Hefner rotor, ONERA HAD-1 propeller, and NASA Dragonfly Phase B* coaxial rotor. The lower-fidelity ADM captures wake characteristics consistent with the ALM with up to a 9× speedup. The ADM maintains strong agreement with experimental and numerical results for integrated performance metrics and is suitable for two-way coupled simulations. The developed framework is applied to a rotor-obstacle configuration based on a GARTEUR 22 experimental survey and subsequently to a fully-coupled ship-rotor-ABL system.
Engineers have developed a next-generation wearable system that enables people to control machines using everyday gestures — even while running, riding in a car, or floating on turbulent ocean waves.
To address the growing concern of increasing noise levels in urban areas, modern automotive vehicles need improved engineering solutions. The need for automotive vehicles to have a low acoustic signature is further emphasized by local regulatory requirements, such as the EU's regulation 540/2014, which sets sound level limits for commercial vehicles at 82 dB(A). Moreover, external noise can propagate inside the cabin, reducing the overall comfort of the driver, which can have adverse impact on the driving behavior, making it imperative to mitigate the high noise levels. This study explores the phenomenon of change in acoustic behavior of external tonal noise with minor geometrical changes to the A-pillar turning vane (APTV), identified as the source for the tonal noise generation. An incompressible transient approach with one way coupled Acoustics Wave solver was evaluated, for both the baseline and variant geometries. Comparison of CFD results between baseline and variant showed spectral broadening of critical tone in variant case. Impact of various other simulation parameters like turbulence intensity, turbulence length scale, time-step size and sampling time, on the critical tonal frequency, was also evaluated. Reduction in time step had a significant impact on the acoustic behavior of the APTVs due to spectral broadening & reduction of tonality. Whereas turbulence intensity is observed to have a significant effect on the frequency of the critical tone, the effect of other simulation parameters was not significant. Coherent vortex shedding from the APTV is identified to be the underlying source of the noise, exhibiting a dipole acoustic behavior. Geometric modification to the leading edge of the APTV is observed to reduce the tonal amplitude due to reduced coherence of vortex shedding and weak vortex core. The current method is able to predict the change in acoustic behavior due to geometric modifications for a particular yaw angle, further studies are ongoing to improve accuracy for full yaw sweep.
Electric Vertical Take-Off and Landing (eVTOL) aircraft, conceptualized to be used as air taxis for transporting cargo or passengers, are generally lighter in weight than jet-fueled aircraft, and fly at lower altitudes than commercial aircraft. These differences render them more susceptible to turbulence, leading to the possibility of instabilities such as Dutch-roll oscillations. In traditional fixed-wing aircraft, active mechanisms used to suppress oscillations include control surfaces such as flaps, ailerons, tabs, and rudders, but eVTOL aircraft do not have the control surfaces necessary for suppressing Dutch-roll oscillations.
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 from the rotor is distributed through the local environment and highlights that, in certain areas, calculated air velocities are found to be at levels considered hazardous to pedestrians.
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 quantities. Results demonstrated that the computational frameworks effectively captured key flow physics, although some discrepancies in drag harmonics, wake velocity and turbulence intensity magnitudes were observed. Additionally, the study highlighted the impact of rotor hub geometry and installation of sail-fairing on drag and wake structures. These findings contribute to improve computational predictions, essential for designing high-speed rotor hub configurations.
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 their periodicity, explaining discrepancies with prior spectral analyses. Overall, the momentum source term approach offers a practical and effective tool for modeling atmospheric turbulence and gust conditions in urban air mobility and rotorcraft simulations.
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, the CETI models for the multi-rotor UAS configurations showed closer alignment with flight data, likely reflecting more accurately captured atmospheric inputs during simulation. These findings highlight the promise of simulation-based CETI modeling for both rotorcraft and UAS, offering valuable insights into turbulence effects across diverse platforms and flight regimes.
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.
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. The computational method was used to simulate the propeller at various angles of attack in low-speed edgewise flight and captured trends and spectral content up to 1 kHz. The predictions showed noise sources moving from root to tip as the propeller angle of attack increased. However, the high-frequency content was not captured for all cases. The experimental campaign was successful in characterizing the acoustics of the Joby Aviation propeller, but more research is needed to be able to properly predict and understand the noise sources throughout the flight envelope.
This study evaluates the effectiveness of two hybrid computational aeroacoustic methods—Lighthill wave model and perturbed convective wave model—in simulating HVAC duct noise in the automotive industry. Using component-level acoustic testing of a Ford HVAC duct, simulations were conducted at varying airflow rates to assess the accuracy of both models in predicting duct noise. The Lighthill wave model, suitable for noise analysis in regions outside turbulent flow areas, showed a good correlation with experimental data, especially in the frequency range of 100 Hz–5000 Hz, but sometimes struggled with pseudo-noise effects at low frequencies near turbulent regions. The perturbed convective wave model, which is suitable for noise analysis anywhere in the flow domain, underpredicted sound pressure levels at low frequencies as well. Both models underpredicted high-frequency noise (>5 kHz) due to insufficient mesh and time-step sizes. Despite these limitations, the Lighthill wave model provided better overall agreement with experimental results. The analysis also identified a resonance peak at 160 Hz, captured in both simulations and experimental data, offering valuable insights for further optimization. These findings underscore the potential of hybrid aeroacoustic models to improve the design of quieter HVAC systems, enhancing both vehicle comfort and customer satisfaction.
In traffic scenarios, the spacing between vehicles plays a key role, as the actions of one vehicle can significantly impact others, particularly with regards to energy conservation. Accordingly, modern vehicles are equipped with inter-vehicle communication systems to maintain specific distances between vehicles. The aerodynamic forces experienced by both leading vehicles (leaders) and following vehicles (followers) are connected to the flow patterns in the wake region of the leaders. Therefore, improving our understanding of the turbulent characteristics associated with vehicles platooning is important. This paper investigates the effects of inter-vehicle distances on the flow structure of two vehicles: a small SUV as the leader and a larger light commercial van as the follower, using a Delayed Detached Eddy Simulation (DDES) CFD technique. The study focuses on three specific inter-vehicle distances: S = 0.28 L, 0.4L, and 0.5L, where S represents the spacing between the two vehicles and L is the length of the leader. Realistic flow conditions are simulated with an average velocity of 31.3 m/s. A comprehensive analysis is conducted by studying the influence of various yaw angles: 0°, -3° and -6°, each representing the vehicle’s alignment with the flow, and effects of 0.33m and 0.66m vehicles’ offsets. This study represents the correlation between the vehicle’s orientation and the aerodynamic forces. The findings indicate the unique flow characteristics at various inter-vehicle distances. These results are then compared to a scaled model tested in a wind tunnel at different inter-vehicle distances. The study demonstrates that changing the vehicle distance results in variations in the length of the recirculation region and flow characteristics behind the vehicles, subsequently impacting the drag and lift coefficients of the leader and the follower. In addition, within a specific range of vehicle distances, the two vehicles can benefit from platooning in terms of drag reduction and consequently less energy consumption. The study also investigates the drag coefficients of both the leader and follower at different yaw angles and vehicles’ offsets. The results highlight that drag coefficients increase at higher yaw angles. Furthermore, the paper shows the distributions of mean velocity, static pressure, turbulence characteristics and 3D vortical structures around the leader and the follower. These results provide valuable information of the complex flow behavior and improve our understanding of the aerodynamic forces around the vehicles during platooning. Such information helps the ongoing efforts to optimize vehicles’ energy consumption.
The vehicle wake region is of high importance when analyzing the aerodynamic performance of a vehicle. It is characterized by turbulent separated flow and large low-pressure regions that contribute significantly to drag. In some cases, the wake region can oscillate between different modes which can pose an engineering challenge during vehicle development. Vehicles that exhibit bimodal wake behavior need to have their drag values recorded over a sufficient time period to take into account the low frequency shift in drag signal, therefore, simulating such vehicle configurations in CFD could consume substantial CPU hours resulting in an expensive and inefficient vehicle design iterations process. As an alternative approach to running simulations for long periods of time, the impact of adding artificial turbulence to the inlet on wake behavior and its potential impact on reduced runtime for design process is investigated in this study. By adding turbulence to the upstream flow, the wake can be prevented from settling into a particular mode and will instead rapidly cycle between modes, which may not only allow the drag to converge to a steady average value in a much shorter simulation time but also prevents bimodal drag behavior from going undetected in the form of early simulation convergence and inconsistent variation across different vehicles. With a more reliable average drag value output from simulations, informed decisions on vehicle configuration can be made during the development phase.
The thermoelectric generator system is regarded as an advanced technology for recovering waste heat from automotive exhaust. To address the issue of uneven temperature distribution within the heat exchanger that limits the output performance of the system, this study designs a novel thermoelectric generation system integrated with turbulence enhancers. This configuration aims to enhance convective heat transfer at the rear end of the heat exchanger and improve overall temperature uniformity. A multiphysics coupled model is established to evaluate the impact of the turbulence enhancers on the system's temperature distribution and electrical output, comparing its performance with that of traditional systems. The findings indicate that the integration of turbulence enhancers significantly increases the heat transfer rate and temperature uniformity at the rear end of the heat exchanger. However, it also leads to an increase in exhaust back pressure, which negatively affects system performance. At lower exhaust flow velocities, the gains in output power attributable to the turbulence enhancers considerably outweigh the increases in exhaust back pressure. Specifically, under conditions of 550 K and 20 m/s, the output power, net output power, and temperature uniformity coefficient increase by 39.2%, 33.6%, and 8.5%, respectively. As exhaust temperature rises, the gains from the turbulence enhancers become even more pronounced. Nevertheless, under high flow conditions, the rise in exhaust back pressure can potentially degrade the system's net output performance. Therefore, it is recommended that exhaust flow be appropriately diverted in practical applications to ensure optimal performance. This research provides essential theoretical guidance for the design and performance optimization of automotive thermoelectric generation systems.
Researchers at Caltech took an important step toward using reinforcement learning to adaptively learn how turbulent wind can change over time, and then uses that knowledge to control a UAV based on what it is experiencing in real time. California Institute of Technology, Pasadena, CA In nature, flying animals sense coming changes in their surroundings, including the onset of sudden turbulence, and quickly adjust to stay safe. Engineers who design aircraft would like to give their vehicles the same ability to predict incoming disturbances and respond appropriately. Indeed, disasters such as the fatal Singapore Airlines flight this past May in which more than 100 passengers were injured after the plane encountered severe turbulence, could be avoided if aircraft had such automatic sensing and prediction capabilities combined with mechanisms to stabilize the vehicle. Now a team of researchers from Caltech's Center for Autonomous Systems and Technologies (CAST) and NVIDIA has taken an important step toward such capabilities. In a new paper published in the journal NPJ Robotics, the team describes a control strategy they have developed for unmanned aerial vehicles, or UAVs, called FALCON (Fourier Adaptive Learning and CONtrol). The strategy uses reinforcement learning, a form of artificial intelligence, to adaptively learn how turbulent wind can change over time and then uses that knowledge to control a UAV based on what it is experiencing in real time.
Researchers have achieved data rates as high as 424Gbit/s across a 53-km turbulent free-space optical link using plasmonic modulators — devices that uses special light waves called surface plasmon polaritons to control and change optical signals. The new research lays the groundwork for high-speed optical communication links that transmit data over open air or space.
In nature, flying animals sense coming changes in their surroundings, including the onset of sudden turbulence, and quickly adjust to stay safe. Engineers who design aircraft would like to give their vehicles the same ability to predict incoming disturbances and respond appropriately. Indeed, disasters such as the fatal Singapore Airlines flight this past May in which more than 100 passengers were injured after the plane encountered severe turbulence, could be avoided if aircraft had such automatic sensing and prediction capabilities combined with mechanisms to stabilize the vehicle.
This study investigates the flow characteristics in the test section of a model-scale, three-quarters open-jet, closed-loop return wind tunnel equipped with a novel device featuring three subsystems to generate transient yaw, gusts, and turbulence. The effect of each subsystem on the resulting turbulent and unsteady flows is evaluated individually and simultaneously. It is demonstrated that this new turbulence generation system can generate yaw distributions with standard deviations ranging from 2.1° to 8.0°. This replicates a wide range of on-road yaw behavior. Additionally, the subsystems can activate transient yaw events and unsteady gusts. Frequency sweeping was demonstrated to fill a wide range of low-frequency spectra, which helps recreate the on-road flow spectra in wind tunnels. Unsteady gusts of more than 15% of the mean flow velocity were achieved. The active turbulence subsystem generates turbulence levels from a few percent, passively, to over 20% intensity levels actively, with tailorable levels depending on input parameters to the active grid. Combined, the subsystems were demonstrated to achieve a wide range of yaw distributions with different standard deviations and features of the on-road turbulence spectrum, from low-frequency events to broadband turbulence with significant inertial subrange within the model-scale wind tunnel.
When traveling in an open-jet wind tunnel, the path of an acoustic wave is affected by the flow causing a shift of source positions in acoustical maps of phased arrays outside the flow. The well-known approach of Amiet attempts to correct for this effect by computing travel times between microphones and map points based on the assumption that the boundary layer of the flow, the so-called shear layer, is infinitely thin and refracts the acoustical ray in a conceptually analogy to optics. However, in reality, the turbulent nature of both the not-so-thin shear layer and the acoustic emission process itself causes an additional smearing of sources in acoustic maps, which in turn causes deconvolution methods based on these maps – the most prominent example being CLEAN-SC – to produce certain ring effects, so-called halos, around sources. In this paper, we intend to cast some light on this effect by describing our path of analyzing/circumventing these halos and how they are linked to the CLEAN algorithm itself. Moreover, we outline a methodological extension to CLEAN-SC, which comes at a reasonable computational cost but effectively eliminates this effect in real-world measurements.
A structural load estimation methodology was developed for RLV-TD HEX-01 hypersonic experimental mission, the maiden winged body technology demonstrator vehicle of ISRO. Primarily the method evaluates time history of station loads considering effects of vehicle dynamics and structural flexibility. Station loads of critical structures are determined by superposition of quasi-static aerodynamic loads, dynamic inertia loads, control surface loads and propulsion loads based on actual physics of the system, improving upon statistical load combination approaches. The technique characterizes atmospheric regime of flight from vehicle loads perspective and ensures adequate structural margin considering atmospheric variations and system level perturbations. Features to estimate change in loads due to wind variability and atmospheric turbulence are incorporated into the load estimation methodology. Augmentation in loads due to structural flexibility is assessed along the trajectory using vehicle states calculated by a flexible vehicle response solver integrated to 6-DOF trajectory solver. This methodology was used to authorize the successful maiden launch of RLV-TD HEX-01 flight on May 23, 2016. The paper describes basic formulation of elastic load estimation technique, pertinent mission design simulation studies and comparison of estimated load with flight measured data. A close agreement between computed structural load and flight measured load throughout the atmospheric regime of flight demonstrates technological maturity of the methodology.
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 and induces recirculating flow in the center of the four rotors. The outwash pattern is non-uniform with jets developing between the rotors and pointing in radially outward directions. Forward flight cases result in a skewed, rapidly merging wake flanked by the roll-up of two "super-vortices" similar to the wingtip vortices of fixed-wing vehicles.
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 command and control link, and the need to monitor electromagnetic interference during flight operations. The use-case produced evidence of the impact of urban airflows on the stability and response of sRPAS. High wind speeds and turbulence intensities were found in the urban flow field of Montréal. The sRPAS use-case results were used to validate wind-speed and turbulence characteristics from laboratory-scale testing on Canadian cities.
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 in the understanding of the wind speed limit for various RPAS in different flows. The paper shows a clear impact of turbulence level on the maximum safe operating wind speed of RPAS.
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 model requires very fine grid in order to predict the expected behavior while transition-criteria approach is less affected by the grid resolution. In terms of rotor performance, both approaches predict the experimental measurements, Langter-Menter tending to slightly overestimate the Figure of Merit where transition criteria approach slightly underestimates its value.
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