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NASA NTRS2010

Developments and Validations of Fully Coupled CFD and Practical Vortex Transport Method for High-Fidelity Wake Modeling in Fixed and Rotary Wing Applications

A novel Computational Fluid Dynamics (CFD) coupling framework using a conventional Reynolds-Averaged Navier-Stokes (BANS) solver to resolve the near-body flow field and a Particle-based Vorticity Transport Method (PVTM) to predict the evolution of the far field wake is developed, refined, and evaluated for fixed and rotary wing cases. For the rotary wing case, the RANS/PVTM modules are loosely coupled to a Computational Structural Dynamics (CSD) module that provides blade motion and vehicle trim information. The PVTM module is refined by the addition of vortex diffusion, stretching, and reorientation models as well as an efficient memory model. Results from the coupled framework are compared with several experimental data sets (a fixed-wing wind tunnel test and a rotary-wing hover test).

Anusonti-Inthra, Phuriwat↗

NASA NTRS1983

The wake model for azimuthal brightness variations in Saturn's A-ring

A dynamical model offering a possible explanation for azimuthal brightness variations in Saturn's ring A is considered. The orbits of ring particles encountering a massive moonlet (radius = about 100 m) are integrated numerically. It is seen that a density wake is produced if the random velocities of the ring particles are much smaller than 1 cm/s.

Karttunen, H.↗

NASA NTRS1981

An extension of the local momentum theory to a distorted wake model of a hovering rotor

The local momentum theory is based on the instantaneous balance between the fluid momentum and the blade elemental lift at a local station in the rotor rotational plane. Therefore, the theory has the capability of evaluating time wise variations of air loading and induced velocity distributions along a helicopter blade span. Unlike a complex vortex theory, this theory was developed to analyze the instantaneous induced velocity distribution effectively. The boundaries of this theory and a computer program using this theory are discussed. A concept introduced into the theory is the effect of the rotor wake contraction in hovering flight. A comparison of this extended local momentum theory with a prescribed wake vortex theory is also presented. The results indicate that the extended local momentum theory has the capability of achieving a level of accuracy similar to that of the prescribed wake vortex theory over wide range variations of rotor geometrical parameters. It is also shown that the analytical results obtained using either theory are in reasonable agreement with experimental data.

Kawachi, K.↗

NASA NTRS1988

Wake model for helicopter rotors in high speed flight

Two alternative approaches are developed to calculate blade-vortex interaction airloads on helicopter rotors: second order lifting-line theory, and a lifting surface theory correction. The common approach of using a larger vortex core radius to account for lifting-surface effects is quantified. The second order lifting-line theory also improves the modeling of yawed flow and swept tips. Calculated results are compared with wind tunnel measurements of lateral flapping, and with flight test measurements of blade section lift on SA349/2 and H-34 helicopter rotors. The tip vortex core radius required for good correlation with the flight test data is about 20 percent chord, which is within the range of measured viscous core sizes for helicopter rotors.

Johnson, Wayne R.↗

NASA NTRS1989

Comparison of measured induced velocities with results from a closed-form finite state wake model in forward flight

An unsteady, three-dimensional induced-flow model is applied to compute the induced-flow distribution of a rotor in forward flight, and numerical results are compared against LDV measurements for both time-averaged and unsteady induced flow at the disk. The former flow shows good agreement with measured data, except just behind the pylon at the lowest advanced ratio and near the blade tips for rectangular blades at high advance ratios. The method performs as well or better than other codes that have been applied to these data, takes less computing time, and is better suited to aeroelastic analysis. Results with only four harmonics and 15 state variables converge to all fundamental characteristics of the time-averaged flow.

Peters, David A.↗

NASA NTRS1992

Extension and validation of an unsteady wake model for rotors

A new three-dimensional, finite-state induced-flow model is extended to treat nonlinearities associated with the mass flow induced through the rotor plane. This new theory is then applied to the correlation of a recent set of unsteady, hover laser Doppler velocimetry inflow measurements conducted in the Aeroelastic Rotor Test Chamber at Georgia Institute of Technology. Although the model is intended primarily as a representation of unsteady aerodynamics for aeroelasticity applications, the results show that it has an excellent capability in predicting the inflow distribution in hover except near the root and tip. In addition, the computed unsteady spanwise lift distribution of a rotor is compared with that from an unsteady vortex lattice method for pitch oscillations at various frequencies. The new model is shown to be capable of prediction of unsteady loads typical of aeroelastic response.

Su, AY↗

DOE OSTI2024

Error analysis of low-fidelity models for wake steering based on field measurements

The observations collected by two scanning lidars deployed on the roof of a 2.8-MW turbine undergoing a series of imposed yaw offsets are analyzed. The wake lateral displacement detected by the rear-facing lidar correlates well with the yaw offset sensed by the forward-facing lidar. We find that the high-frequency part of the yaw offset signal is connected to wake meandering, whereas the low frequency component is a good predictor for wake displacement due to yaw misalignment. Conditionally averaged wake velocity data for different yaw offsets are used as benchmarks for the validation of a linearized Reynolds-averaged Navier-Stokes and an empirical wake model. A mean error as low as 2% and a good prediction of the wake trajectory are achieved, provided that the wake recovery rate matches the observations.

17 WIND ENERGY↗

NASA NTRS2016

A Coupled Probabilistic Wake Vortex and Aircraft Response Prediction Model

Wake vortex spacing standards along with weather and runway occupancy time, restrict terminal area throughput and impose major constraints on the overall capacity and efficiency of the National Airspace System (NAS). For more than two decades, the National Aeronautics and Space Administration (NASA) has been conducting research on characterizing wake vortex behavior in order to develop fast-time wake transport and decay prediction models. It is expected that the models can be used in the systems level design of advanced air traffic management (ATM) concepts that safely increase the capacity of the NAS. It is also envisioned that at a later stage of maturity, these models could potentially be used operationally, in groundbased spacing and scheduling systems as well as on the flight deck.

Gloudemans, Thijs↗

NASA NTRS1988

Development and validation of an advanced low-order panel method

A low-order potential-flow panel code, PMARC, for modeling complex three-dimensional geometries, is currently being developed at NASA Ames Research Center. The PMARC code was derived from a code named VSAERO that was developed for Ames Research Center by Analytical Methods, Inc. In addition to modeling potential flow over three-dimensional geometries, the present version of PMARC includes several advanced features such as an internal flow model, a simple jet wake model, and a time-stepping wake model. Data management within the code was optimized by the use of adjustable size arrays for rapidly changing the size capability of the code, reorganization of the output file and adopting a new plot file format. Preliminary versions of a geometry preprocessor and a geometry/aerodynamic data postprocessor are also available for use with PMARC. Several test cases are discussed to highlight the capabilities of the internal flow model, the jet wake model, and the time-stepping wake model.

Ashby, Dale L.↗

NASA NTRS2014

Review of Idealized Aircraft Wake Vortex Models

Properties of three aircraft wake vortex models, Lamb-Oseen, Burnham-Hallock, and Proctor are reviewed. These idealized models are often used to initialize the aircraft wake vortex pair in large eddy simulations and in wake encounter hazard models, as well as to define matched filters for processing lidar observations of aircraft wake vortices. Basic parameters for each vortex model, such as peak tangential velocity and circulation strength as a function of vortex core radius size, are examined. The models are also compared using different vortex characterizations, such as the vorticity magnitude. Results of Euler and large eddy simulations are presented. The application of vortex models in the postprocessing of lidar observations is discussed.

Ahmad, Nashat N.↗

NASA NTRS1989

The influence of the rotor wake on rotorcraft stability and control

The effect of the time-averaged rotor wake flowfield on the aerodynamic behavior of the tail rotor and fixed tail surfaces is discussed. The flowfield at the location of these surfaces is predicted by two wake models, a simplified flat wake model and an accurate free wake model. Both models are shown to give similar predictions of the flowfield in the vicinity of the empennage that are generally in agreement with experiment. The contributions of these aerodynamic interactions to the helicopter stability derivatives are described and control responses using different wake models are compared with flight test.

Curtiss, H. C., Jr.↗

NASA NTRS2015

Multi-Model Ensemble Wake Vortex Prediction

Several multi-model ensemble methods are investigated for predicting wake vortex transport and decay. This study is a joint effort between National Aeronautics and Space Administration and Deutsches Zentrum fuer Luft- und Raumfahrt to develop a multi-model ensemble capability using their wake models. An overview of different multi-model ensemble methods and their feasibility for wake applications is presented. The methods include Reliability Ensemble Averaging, Bayesian Model Averaging, and Monte Carlo Simulations. The methodologies are evaluated using data from wake vortex field experiments.

Koerner, Stephan↗

NASA NTRS2015

Multi-Model Ensemble Wake Vortex Prediction

Koerner, Stephan↗

DOE OSTI2022

Towards multi-fidelity deep learning of wind turbine wakes

We report engineering wake models that accurately predict wake in a computationally efficient manner are very important for tasks such as layout optimization and control of wind farms. In this paper, we explore an application of deep learning (DL) to learn the wake model from hierarchies of physics-based approaches ranging from analytical models to an approximate form of the Reynolds-averaged Navier-Stokes equations. We first illustrate the application of principal component analysis to obtain a lower-dimensional representation that allows a computationally tractable training and deployment of DL models. Then, the DL model is trained to learn the mapping from input parameter space to the principal components, which are then used to reconstruct the three-dimensional flow field. Additionally, we investigate a composite framework consisting of two neural networks to learn the correlation between low- and high-fidelity data with Gauss and curl models treated as proxies for low- and high-fidelity models, respectively. The prediction from both DL models matches well with the high-fidelity data with a maximum relative percentage error for the kinetic energy flux of <1%. This work opens up possibilities for data-efficient construction of surrogate models for wake prediction that can be used to study the influence of wind speed and yaw angles on wind farm power production.

17 WIND ENERGY↗

NASA NTRS1989

Evaluation of VSAERO in prediction of aerodynamic characteristics of helicopter hub fairings

A low-order panel code, VSAERO, was used to predict the aerodynamic characteristics of helicopter hub fairings. Since the simulation of this kind of bluff body by VSAERO was not documented before, the VSAERO solutions were correlated with experimental data to establish their validity. The validation process revealed that simulation of the aerodynamic environment around a hub fairing was sensitive to several modeling parameters. Some of these parameters are body and wake panels arrangement, streamwise and spanwise separation location, and the most prominent one-the wake modeling. Three wake models were used: regular wake, separated wake, and jet model. The regular wake is a wake with negligible thickness (thin wake). It is represented by a single vortex sheet. The separated wake and the jet model in the present application are wakes with finite thickness (thick wake). They consist of a vortex sheet enclosing a region of low-energy flow. The results obtained with the reqular wake were marginally acceptable for sharp-edged hub fairings. For all other cases under consideration, the jet model results correlated slightly better. The separated wake, which seemed to be the most appropriate model, caused the solution to diverge. While the regular wake was straight-forward to apply in simulations, the jet model was not. It requires the user to provide information about the doublet strength gradient on wake panels by guessing the efflux velocities at the wake shedding location. In summary, VSAERO neither predicts accurately the aerodynamic characteristics of helicopter hub fairings nor was cost effective.

Louie, Alexander↗

NASA NTRSDate not supplied

A Physics-Based Spatial Wake Interactional Model of Fixed-Wing Aircraft and Rotorcraft for Urban Air Mobility

This paper presents a study of a physics-based spatial wake interactional model for urban air mobility. A modeling method for spatial wake interaction of fixed-wing aircraft and rotorcraft is presented. The method validates an existing wake vortex model with flight data from the existing literature. The model is then adopted for both fixed-wing aircraft and rotorcraft. A lift circulation model is developed for rotorcraft to account for unsteady aerodynamics and the compressibility effect. This lift circulation model provides more realistic estimate of lift circulation for rotorcraft for used in wake vortex modeling. A vehicle-building wake interaction is studied using the method of image. The effects of wake vortex encounters on aerodynamic loads, stability, and control are examined. A flight control simulation is performed to demonstrate recovery during a simulated wake vortex encounter.

Wake Vortex Encounter↗

DOE OSTI2025

Modeling the effects of active wake mixing on wake behavior through large-scale coherent structures

The use of active wake mixing (AWM) to mitigate downstream turbine wakes has created new opportunities for reducing power losses in wind farms. However, many current analytical or semi-empirical wake models do not capture the flow instabilities that are excited through the blade pitch actuation. In this work, we develop a framework, which accounts for the impacts of the large-scale coherent structures and turbulence on the mean flow, for modeling AWM. The framework uses a triple-decomposition approach for the unsteady flow field and models the mean flow and fine-scale turbulence with a parabolized Reynolds-averaged Navier–Stokes (RANS) system. The wave components are modeled using a simplified spatial linear stability formulation that captures the growth and evolution of the coherent structures. Comparisons with high-fidelity large eddy simulations (LESs) of the turbine wakes showed that this framework was able to capture the additional wake mixing and faster wake recovery in the far-wake regions for both the pulse and helix AWM strategies with minimal computational expense. In the near-wake region, some differences are observed in both the RANS velocity profiles and initial growth of the large-scale structures, which may be due to some simplifying assumptions used in the model.

17 WIND ENERGY↗

NASA NTRS2019

Correlation of Full-Scale Isolated Proprotor Performance and Loads

A full-scale isolated proprotor test was recently conducted in the USAF National Full-Scale Aerodynamics Complex (NFAC) at NASA Ames Research Center. The test article was a 3-bladed research rotor derived from the right-hand rotor of the AW609. For this test, the NASA Tiltrotor Test Rig (TTR) and rotor were installed in the 40- by 80-foot test section. This paper presents correlations between data and predictions of rotor performance and blade moments using the newly acquired test data and the comprehensive analysis CAMRAD II. The operational conditions covered in this analytical study are: hover (actually, low speed vertical climb), cruise (airplane mode), conversion, and helicopter mode. Mean and 1/2 peak-to-peak quantities (hpp) are correlated; time-history correlation for the helicopter condition is also included. The correlation is reasonable to good. Also, the hover calculations turned out to be useful in providing reality checks on the test hardware such as: a) the functioning of the blade strain gages and b) calibration of the measurement of the collective pitch hardware. The time-history correlation shows that, compared to the rolled-up wake model, the multiple-trailer wake model improves the correlation slightly; the longitudinal cyclic correlation is reasonable but the lateral cyclic correlation is not good, and the collective is predicted well by the rolled-up wake model; the flap moment correlation is reasonable; the pitch link load and lag moment are underpredicted; and the torsion moment correlation is poor and needs further study.

Kottapalli, Sesi↗