Aerodynamic Method Research Articles

Quantifying the Aerodynamic Roughness Length of Snow Surfaces With Time‐Lapse Structure‐From‐Motion

AbstractThe aerodynamic roughness length (z0m) of snow surfaces plays an essential role in estimating turbulent fluxes of latent and sensible heat (LE and H). Constant z0m has been widely used in previous studies due to the difficulty in monitoring dynamic z0m. This leads to large uncertainties in simulating LE and H fluxes. In the current study we used a time‐lapse structure‐from‐motion (SfM) photogrammetric method to study temporal variation in z0m of snow surfaces. The study site was located at the edge of the August‐one ice cap in the Qilian Mountains of China. The study ran from August 2019 to September 2020. We measured aerodynamic roughness length over time (z0m_t) using SfM‐based micro‐topographic methods and validated our results by comparing z0m_t with z0m_prof (profile‐based) and z0m_EC (EC‐based) data. The validated dynamic z0m_t was then used to estimate LE and H fluxes using a bulk aerodynamic method. These results were then compared with LE and H measured by eddy covariance (EC) and with manual sublimation/condensation measurements to indirectly validate z0m_t. We found that z0m_t overall agreed with profile‐based z0m_prof and EC‐based z0m_EC. Indeed, our estimates of bulk LE and H fluxes using dynamic z0m_t agreed slightly better with EC and manual sublimation/condensation measurements than did static EC‐based z0m_EC. Micro‐topographic z0m_t can be helpful in precisely estimating varying LE and H fluxes over the course of a year.

Proposal of a Method of Aerodynamic Force Measurement of Aircraft Model without Using Force Sensors in Wind Tunnel Testing

Recently, the use of unmanned air vehicles (UAV) at disaster sites has increased, and research and development has been active. Since these vehicles fly at low Reynolds number, they have different aerodynamic characteristic from conventional aircraft. Therefore, it is expected that research as flight test, wind tunnel tests and CFD will be accelerated in order to improve aerodynamic performance in this area. This research focused on Hand Launch Glider with high performance in the Re=104 regions. This paper proposes a new aerodynamic force measurement method based on positional measurements suitable for wind tunnel tests on a full-scale model of a Hand Launch Glider. Validation and experimentation of this method were conducted by measuring the lift and drag forces generated on the aircraft every A.O.A. and comparing the results of Koike et al. The results show that although the accuracy of the model's weight measurement has a significant effect on the aerodynamic force measurement, the validity of the new aerodynamic force measurement method based on positional measurements was considered. The proposed new aerodynamic force measurement method is available for evaluating the aerodynamic characteristics of slender fuselage airframes which cannot be inserted force sensors, or small loads that make it preciously difficult to measure aerodynamic forces with sensors, like the Hand Launch Glider.

Computational aerodynamics with isogeometric analysis

AbstractThe superior accuracy isogeometric analysis (IGA) brought to computations in fluid and solid mechanics has been yielding higher fidelity in computational aerodynamics. The increased accuracy we achieve with the IGA is in the flow solution, in representing the problem geometry, and, when we use the IGA basis functions also in time in a space–time (ST) framework, in representing the motion of solid surfaces. It is of course as part of a set of methods that the IGA has been very effective in computational aerodynamics, including complex-geometry aerodynamics. The set of methods we have been using can be categorized into those that serve as a core method, those that increase the accuracy, and those that widen the application range. The core methods are the residual-based variational multiscale (VMS), ST-VMS and arbitrary Lagrangian–Eulerian VMS methods. The IGA and ST-IGA are examples of the methods that increase the accuracy. The complex-geometry IGA mesh generation method is an example of the methods that widen the application range. The ST Topology Change method is another example of that. We provide an overview of these methods for IGA-based computational aerodynamics and present examples of the computations performed. In computational flow analysis with moving solid surfaces and contact between the solid surfaces, it is a challenge to represent the boundary layers with an accuracy attributed to moving-mesh methods and represent the contact without leaving a mesh protection gap.

A Combined Artificial-Intelligence Aerodynamic Design Method for a Transonic Compressor Rotor Based on Reinforcement Learning and Genetic Algorithm

An aircraft engine’s performance depends largely on the compressors’ aerodynamic design, which aims to achieve higher stage pressure, efficiency, and an acceptable stall margin. Existing design methods require substantial prior knowledge and different optimization algorithms to determine the 2D and 3D features of the blades, in which the design policy needs to be more readily systematized. With the development of artificial intelligence (AI), deep reinforcement learning (RL) has been successfully applied to complex design problems in different domains and provides a feasible method for compressor design. In addition, the applications of AI methods in compressor research have progressively developed. This paper described a combined artificial-intelligence aerodynamic design method based on a modified deep deterministic policy gradient algorithm and a genetic algorithm (GA) and integrated the GA into the RL framework. The trained agent learned the design policy and used it to improve the GA optimization result of a single-stage transonic compressor rotor. Consequently, the rotor exhibited a higher pressure ratio and efficiency owing to the sweep feature, lean feature, and 2D airfoil angle changes. The separation near the tip and the secondary flow decreased after the GA process, and at the same time, the shockwave was weakened, providing improved efficiency. Most of these beneficial flow field features remained after agent modification to improve the pressure ratio, showing that the policy learned by the agent was generally universal. The combination of RL and other design optimization methods is expected to benefit the future development of compressor designs by merging the advantages of different methods.

Dry deposition of ammonia in a coastal dune area: Measurements and modeling

Ammonia deposition is a threat to many natural ecosystems, including coastal dune areas, because of eutrophication and acidification. Direct measurements of ammonia fluxes are nevertheless scarce. In this paper we present a full year of measurements to derive the ammonia dry deposition flux in a Dutch coastal dune ecosystem, based on the aerodynamic flux-gradient method (AGM). We found a mean ammonia flux of −7.1 ± 1.7 ng m−2 s−1, and an annual ammonia deposition flux of −132 ± 32 mol ha−1 yr−1 (equivalent to 1.8 ± 0.4 kg N ha−1 yr−1), which is at the low end of the range from estimates from literature made with inferential methods. Modeling the fluxes with the DEPAC module resulted in a mean flux of −17.0 ng m−2 s−1. The model overestimated the deposition fluxes, but diurnal variations of the fluxes derived from measurements were well captured by the model. We propose to change certain DEPAC parameters, like the leaf area index, to values more applicable for a dune ecosystem and show that this improves the agreement between model and measurements.

The impact of the wind attack angle on a typical bridge deck’s flutter behavior by the distributed aerodynamic characteristics method

To investigate the influence of the wind attack on a bridge deck’s flutter performance, the concept of “pressure flutter derivatives” was introduced as a part of the “distributed aerodynamic characteristics” method, which can be used to visualize the dominant fluctuations of the pressure field. The distributions of the aerodynamic damping and stiffness under different wind attack angles were visualized, instead of using an overall force perspective. The analysis was carried out through two-dimensional computational fluid dynamics simulations based on a typical box deck section. Necessary data were obtained through the forced vibrations of the deck. The dominant pressure fluctuations were demonstrated, around which the wind attack angles’ impact on the Scanlan flutter derivatives was discussed. For the deck section used in this study, the wind attack angle was found to most affect the windward parts of the deck. When wind attack angle ranged from − 1.5 ° to 3 ° , two regions contributed the most: one located behind the windward faring vertex and the other located behind the first vertex of the upper slab. When the wind attack angle decreased to − 3 ° , a new dominant region appeared behind the first vertex of the lower slab, which caused a sudden reduction of the flutter critical speed.

Research on Adaptive Prescribed Performance Control Method Based on Online Aerodynamics Identification

Wide-speed-range vehicles are characterized by high flight altitude and high speed, with significant changes in the flight environment. Due to the strong uncertainty of its aerodynamic characteristics, higher requirements are imposed on attitude control. In this paper, an adaptive prescribed performance control method based on online aerodynamic identification is proposed, which consists of two parts: an online aerodynamic parameter identification method and an adaptive attitude control method based on the pre-defined parameters of the control system. The aerodynamic parameter identification is divided into offline design and online design. In the offline design, neural networks are used to fit nonlinear aerodynamic characteristics. In the online design, a nonlinear recursive identification method is used to correct the errors of the offline fitted model. The adaptive attitude control is based on the conventional control method and updates the control gain in real time according to the desired system parameters to enhance the robustness of the controller. Finally, the effectiveness of the offline neural network and online discrimination correction is verified by mathematical simulations, and the effectiveness and robustness of the adaptive control proposed in this paper are verified by comparative simulation.

Review on power conversion unit of noble gas closed Brayton cycle for space and underwater applications

The closed Brayton cycle (CBC) has become a promising solution for the high-energy density power conversion unit in underwater and space applications. Differing from the design requirements of the grounded plant, the CBC in the underwater application gives priority to the compact size. Therefore, there is a need to provide a summary of the current research, particularly on working fluid and turbomachinery, with the view to obtaining the strategies adopted in power conversion unit to meet the design requirements of the underwater and space application. This paper presented the comprehensive considerations behind the selection of helium as the ideal working fluids for underwater and space applications and reviews the various predictions methods of the unacquainted physical properties of noble gas, among which the method pertaining to the Chapman-Enskog kinetic theory and the law of the corresponding state is more precise with the comprehensive accuracies of 2% for pure gas and 5% for the binary mixture. The studies on the characteristics of noble gas to support the application feasibility of the existing aerodynamic method to it were also reviewed. Two strategies to reduce the size of the power conversion unit, based on highly-loaded design and He-Xe turbomachinery design, were presented. Through the strategies, the stage number of the compressor under the given pressure ratio can reduce by about 75% compared with the prototype. Moreover, the paper also paid an attention to the similarity modeling method in the performance map of turbomachinery between the noble gas and air. Based on the review, the current problems encountered and the expected future breakthroughs were highlighted.

Computational methods in aerodynamics

Computational methods in aerodynamics

Computational methods in aerodynamics

Computational methods in aerodynamics

РАЗРАБОТКА СТЕНДА ИЗМЕРЕНИЯ ТЯГИ НА ОСНОВЕ АЭРОДИНАМИЧЕСКОГО МЕТОДА ДЛЯ ЭЛЕКТРОРАКЕТНЫХ ДВИГАТЕЛЕЙ МАЛЫХ КОСМИЧЕСКИХ АППАРАТОВ

A traction measurement stand based on the aerodynamic method has been developed. To test the stand, a prototype electric rocket engine was used as a cold gas engine. The working medium was nitrogen gas. At a flow rate of 1.2 mg/s, the thrust value was from 0.674 to 0.736 mN, at a flow rate of 1.7 mg/s, the thrust value was 0.934-1 mN and at a flow rate of 2.6 mg/s, the thrust value was from 1.59 to 1.634 mN. The experimental specific thrust impulse was at a flow rate of 1.2 mg/s from 574 to 613 m/s, at a flow rate of 1.7 mg/s from 549 to 588 m/s and at a flow rate of 2.6 mg/s from 612 to 623 m/s. According to the measured pressure values in the chamber of the prototype electric rocket engine, the values of the ideal flow rate of the working fluid from the nozzle were obtained, which amounted to 661 m/s at a flow rate of 1.2 mg/s, 667 m/s at a flow rate of 1.7 mg/s and 674 m/s at a flow rate of 2.6 mg/s. The obtained values of the specific thrust impulse do not contradict the previously obtained experimental data on cold gas jet engines using nitrogen as the working fluid, and also do not exceed the ideal design flow rate

Thermophysical properties of molten (Fe2O3)0.95-(SiO2)0.05 measured by aerodynamic levitation

In order to establish an evaluation method/numerical simulation for nuclear reactor safety under severe accidental conditions, it is necessary to obtain the physical properties of the relevant molten materials at very high temperatures. In particular, the reaction/interaction between the melt of stainless-steel oxide originating from the nuclear reactor component and the composition of structural concrete is an important phenomenon in terms of understanding of the progress of severe accidents in nuclear power plants and the planning/installation of equipment/devices as countermeasures. The installation of a core catcher is one possible countermeasure to safely terminate a severe accident. For this to work a sacrificial material is placed in the core catcher to increase the fluidity of the molten material. Iron oxide (Fe2O3) is considered a promising candidate. In this study, thermophysical properties such as the density and the viscosity of a (Fe2O3)0.95-(SiO2)0.05 mixture were obtained using the aerodynamic levitation method. The chosen composition is representative for the Molten-Core-Concrete-Interaction at early stages of a severe accident event. Although partial Fe2O3 changes to Fe3O4 during the experiment, this composition change would occur under the actual severe accident conditions. The physical property values of the (Fe2O3)0.95-(SiO2)0.05 mixture were almost the same as those of Fe2O3 obtained in an earlier study. Therefore, it can be concluded that the fluidity of Fe2O3 is not significantly affected in the early stages of a severe accident whereby small amounts of SiO2 (approximately 5 mol. %) are dissolved into Fe2O3.

Aeroservoelastic Stability Evaluation for Slender Vehicles Based on the Ground Frequency Response Test

With the increasing bandwidths of servo control systems and decreasing mode frequencies, aeroservoelastic (ASE) stability evaluation has become an essential part of flight vehicle design. However, the theoretical method is limited by the modeling errors of numerical models, and the dry wind tunnel method is limited by the complex design of force controllers. Given these limitations, a novel ASE stability evaluation method for slender vehicles based on the ground frequency response test (FRT) is proposed in this paper. FRTs are implemented for a slender vehicle to obtain the frequency response functions (FRFs) of the real structure and servo control systems. The low-order unsteady aerodynamic FRFs established in physical coordinates are calculated by the quasi-steady aerodynamic derivative method. An ASE open-loop FRF is established for stability evaluation via the Nyquist criterion. Comparison with the theoretical results shows that the proposed method is feasible and accurate for different positions of the inertial measurement unit, different control laws, and different Mach numbers. To deal with the unavoidable influence of hanging supports in the test, an FRF fitting and resynthesis method is used to remove the hanging modes and provide an accurate ASE open-loop FRF with free–free boundary conditions.

An overall blockage attenuation-based aerodynamic performance and stability design optimization method for transonic axial flow compressors

PurposeThis study aims to use computational fluid dynamics (CFD) to understand and quantify the overall blockage within a transonic axial flow compressor (AFC), and to develop an efficient collaborative design optimization method for compressor aerodynamic performance and stability in conjunction with a surrogate-assisted optimization technique.Design/methodology/approachA quantification method for the overall blockage is developed to integrate the effect of regional blockages on compressor aerodynamic stability and performance. A well-defined overall blockage factor combined with efficiency drives the optimizer to seek the optimum blade designs with both high efficiency and wide-range stability. An adaptive Kriging-based optimization technique is adopted to efficiently search for Pareto front solutions. Steady and unsteady numerical simulations are used for the performance and flow field analysis of the datum and optimum designs.FindingsThe proposed method not only remarkably improves the compressor efficiency but also significantly enhances the compressor operating stability with fewer CFD calls. These achievements are mainly attributed to the improvement of specific flow behaviors oriented by the objectives, including the attenuation of the shock and weakening of the tip leakage flow/shock interaction intensity.Originality/valueCFD-based design optimization of AFC is inherently time-consuming, which becomes even trickier when optimizing aerodynamic stability since the stall margin relies on a complete simulation of the performance curve. The proposed method could be a good solution to the collaborative design optimization of aerodynamic performance and stability for transonic AFC.

Aerodynamic Force Control for Tilt-Wing eVTOL Using Airflow Vector Estimation

Research and development are active in electric vertical takeoff and landing (eVTOL) aircraft. In particular, tilt-wing eVTOL aircraft receives much attention as one of the most efficient configurations; however, they are likely to be unstable during the transition from hover to cruise because the lift and thrust have limitations depending on the airflow and tilt angle. This study proposes a new aerodynamic force control method using airflow vector estimation. The airflow vector is estimated by combining motor current, rotational speed, and Pitot-tube measurements. Aerodynamic force control is achieved through the proper design of a feedback controller using disturbance observers (DOBs) to cope with propeller-wing interference caused by the propeller slipstream. This method takes advantage of the motor control performance and is unique in which it monitors the airflow vector and actively changes the tilt angle to quickly obtain the desired acceleration. The effectiveness of the method is verified via simulations and experiments in a wind tunnel.

Enhanced nonlinear state–space identification for efficient transonic aeroelastic predictions

Transonic aerodynamic systems exhibit strong nonlinearities due to various factors such as flow separations and shock wave oscillations. Thus, transonic aerodynamic modeling methods based on system identifications have attracted increasing attention. However, the dimensions of the models constructed via the current system identification methods are so high that it is difficult to predict transonic flutters and limit cycle oscillations simultaneously. This paper proposes an enhanced nonlinear state–space modeling method to identify a two-dimensional aerodynamic system in the transonic region. At first, the parsimonious linear model with modified inputs is identified by using the eigensystem realization algorithm. The model order is reduced by substituting the generalized displacement of the first-order mode (dominated by plunge) with its generalized velocity in the inputs. The polynomial functions are then used to represent the nonlinearities in the transonic aerodynamic systems. The polynomial functions do not contribute to the system linearization around the equilibrium such that the linear and nonlinear modeling stages are independent. The coefficients of these nonlinear terms are determined via nonlinear optimization. Finally, a nonlinear aerodynamic model is established and applied to aeroelastic applications. To demonstrate the accuracy and efficiency of this method, a two-dimensional, transonic aeroelastic wing with an NACA0012 profile is investigated. The simulation results show that the nonlinear state–space model with modified inputs improves the efficiency and accuracy of transonic aerodynamic nonlinear modeling. Moreover, the aerodynamic model coupled with the structural model can accurately predict the transonic flutter boundary and limit cycle oscillations.

Accuracy of methods for simulating daily water surface evaporation evaluated by the eddy covariance measurement at boreal flux sites

Evaporation from the surface of water is a vital component of the hydrological cycle. Accurately simulating evaporation is crucial for modeling hydrological processes, ensuring ecosystem water efficiency, and managing water resources. Previous methods for evaluating evaporation have generally been limited to a single site and on a regional scale. Based on eddy covariance measurements of 14 boreal flux sites, the accuracy of 26 methods for simulating daily water surface evaporation was comprehensively evaluated in this study. Most methods accurately simulated daily evaporation at most sites with a KGE >0.60. The combination methods with simultaneous calibration of the energy and aerodynamic terms and the double-parameter aerodynamic methods based on the linear and exponential functions showed the best performance, with the median KGE of the evaluated sites ranging from 0.72 to 0.76. The Bowen ratio energy balance (BREB) method embedded with Tw, Rs-based two-variable empirical methods, double-parameter aerodynamic methods, and the combination method of the energy term based on net radiation (Rn) and the aerodynamic term based on the exponential function method performed better than the other methods of the same type. The relative error (RE) simulated by most methods was generally within ± 30 %, with the median RE of all sites within ± 10 % for each method. The combination methods tended to overestimate the level of evaporation, whereas the BREB-type and aerodynamic methods tended to underestimate the extent of evaporation. The simulation accuracy of the daily evaporation showed significant variance among the sites. It showed the best model performance at two FLUXNET and two sites taken from the literature, in which 75 % of the methods were able to accurately simulate daily evaporation with a Kling–Gupta efficiency (KGE) >0.80. All methods performed poorly at the three sites from the literature and were mainly due to the lack of measured variables with a strong correlation with the latent heat flux (LE). The FLUXNET sites generally showed better performance. The observed LE of the sites with the best model performance generally showed a unimodal distribution throughout the year. The simulation accuracy of non-unimodal distribution sites mainly depended on the correlation between the LE and related variables. The combination of the energy term based on Rn and the aerodynamic term based on the power function method showed the best model stability. This finding indicated that the calibrated method was robust and showed high stability in simulating daily evaporation, and it was recommended for application in other sites or regions. The optimization of parameters calibrated in this study may improve the accuracy of simulating the daily evaporation for the application of the Penman and Priestley–Taylor equations when the water heat storage measurement is missing.

Fast Prediction of Flow Field around Airfoils Based on Deep Convolutional Neural Network

We propose a steady-state aerodynamic data-driven method to predict the incompressible flow around airfoils of NACA (National Advisory Committee for Aeronautics) 0012-series. Using the Signed Distance Function (SDF) to parameterize the geometric and flow condition setups, the prediction core of the method is constructed essentially by a consecutive framework of a convolutional neural network (CNN) and a deconvolutional neural network (DCNN). Impact of training parameters on the behavior of the proposed CNN-DCNN model is studied, so that appropriate learning rate, mini-batch size, and random deactivation rate are specified. Tested by “unseen” airfoil geometries and far-field velocities, it is found that the prediction process is three orders of magnitudes faster than a corresponding Computational Fluid Dynamics (CFD) simulation, while relative errors are maintained lower than 1% on most of the sample points. The proposed model manages to capture the essential dynamics of the flow field, as its predictions correspond reasonably with the reconstructed field by proper orthogonal decomposition (POD). The performance and accuracy of the proposed model indicate that the deep learning-based approach has great potential as a robust predictive tool for aerodynamic design and optimization.

Aerodynamic validation for compressor blades’ structural morphing concepts

For increasing an aircraft’s engine efficiency and reducing emissions, the use of adaptive blades capable of guaranteeing an optimal performance at different flight phases is researched. An aerodynamic design point blade shape and some exemplary possible morphed shapes for different operational conditions are introduced and analyzed from a structural as well as from an aerodynamic point of view. For this purpose, the structural design process developed to calculate the blade geometries that can be reached through structurally integrated actuation is introduced and explained with the help of three morphing blade example geometries. Furthermore, the aerodynamic methods used for the evaluation of the structurally achieved morphed geometries is also studied with the help of the introduced examples.

Research on the Determination Method of Aircraft Flight Safety Boundaries Based on Adaptive Control

Icing is one of the main external environmental factors causing loss of control (LOC) in aircraft. To ensure safe flying in icy conditions, modern large aircraft are all fitted with anti-icing systems. Although aircraft anti-icing technology is becoming more sophisticated as research continues to expand and deepen, the scope of protection provided by anti-icing systems based on existing anti-icing technology is still relatively limited, and in practice, it is difficult to avoid flying with ice even when the anti-icing system is switched on. Therefore, it is necessary to consider providing additional safety strategies in addition to the anti-icing system, i.e., to consider icing safety from the aerodynamic, stability, and control points of view during the aircraft design phase, and to build a complete ice-tolerant protection system combining aerodynamic design methods, flight control strategies and implementation equipment. Based on the modern control theory of adaptive control, this paper presents a new method of envelope protection in icing situations based on a case study of icing, which has the advantages of strong real-time performance and good robustness, and has high engineering application value.