Supervised learning with probability interpretation in airfoil transition judgment

Transition prediction has always been a frontier issue in the field of aerodynamics.A supervised learning model with probability interpretation for transition judgment based on experimental data was developed in this paper.It solved the shortcomings of the point detection method in the experiment,that which was often only one transition point could be obtained,and comparison of multi-point data was necessary.First,the Variable-Interval Time Average(VITA)method was used to transform the fluctuating pressure signal measured on the airfoil surface into a sequence of states which was described by Markov chain model.Second,a feature vector consisting of one-step transition matrix and its stationary distribution was extracted.Then,the Hidden Markov Model(HMM)was used to pre-classify the feature vectors marked using the traditional Root Mean Square(RMS)criteria.Finally,a classification model with probability interpretation was established,and the cross-validation method was used for model validation.The research results show that the developed model is effective and reliable,and it has strong Reynolds number generalization ability.The developed model was theoretically analyzed in depth,and the effect of parameters on the model was studied in detail.Compared with the traditional RMS criterion,a reasonable transition zone can be obtained using the developed classification model.In addition,the developed model does not require comparison of multi-point data.The developed supervised learning model provides new ideas for the transition detection in flight experiments and other experiments.

Experimental and numerical study on dynamic stall under a large Reynolds number

Dynamic stall under large Reynolds numbers and large reduced frequencies has a significant effect on the performance of the wind turbine blades,helicopter rotors,etc.So the dynamic stall physics of the NACA0012 airfoil under a large Reynolds number of Re=1.5×10^(6) was studied using experimental and numerical methods.The reduced frequency range was k=0.035-0.1.The unsteady flow field in dynamic stall was studied in detail by using the transient pressure measurement and the numerical simulation based on the unsteady Reynolds-averaged Navier-Stokes(URANS)equation.And the time-frequency characteristics of the dynamic stall were studied using the wavelet analysis.The study showed that the aerodynamic performance during the dynamic stall was dominated by the shear layer vortex(SLV)and the leading edge vortex(LEV),and the phase difference between the SLV and the LEV was the key factor in the exist-ence of the bimodal characteristics of the aerodynamic force/moment.There was a significant linear correlation between the negative peak of the vortex-induced Cp and the Cn in the reduced frequency range studied in this paper.During the convection of the near-wall LEV to the trailing edge,the high-frequency features firstly decay,and the multi-scale structures of the LEV become more significant as the reduced frequency gradually increases.

Experimental study on transition of dynamic airfoil in pitching oscillation

The transition characteristics of dynamic airfoil have significant effects on the aerodynamic performance of wind turbines,helicopter rotor blades,jet engine compressor blades,etc.The time domain and time-frequency domain characteristics of transition on a NACA0012 airfoil during its pitching oscillation were experimentally studied using wall pressure measurement technology with high time accuracy in this paper.The vari-able slip window technology was used to detect the transition position,and the proper orthogonal decomposition(POD)method and wavelet analysis were combined to perform the time-frequency analysis.In the gradual forward movement of the transi-tion,the low-frequency instability is gradually enhanced by the main flow and the inverse pressure gradient,and significantly submerges the high-frequency fluctuated feature.The higher order moments of the wall pressure during dynamic airfoil transi-tion deviate significantly from the Gaussian characteristics,which is caused by the low-frequency instability and high-frequency burst.The POD method is able to distinguish low-frequency instability from the high-frequency feature.The reduced frequency had significant effects on the transition.With the increase of the reduced frequency,the hysteresis effect of the transition became more and more significant,and the fre-quency component of transition was more concentrated and the energy was stronger.

Effect of vortex generator spanwise height distribution pattern on aerodynamic characteristics of a straight wing

Vortex generator(VG)is a passive flow control technology,which can effectively inhibit flow separation.Recently,significant research efforts have been devoted to the study on the application of VG in airfoil,wing/blade and aircraft flow separation inhibition.However,the research on the variable height distribution of VG along the wingspan still lacks.This study focuses on the experimental investigation of the influence of variable height distribution of ramp VG on aerodynamic characteristics.First,VG was designed,such that it was composed of two parts,one fixed region and one deformed region,which allowed for height modification.The height of VGs along the wingspan presented equal-height,triangular and trapezoidal distribution,forming a total of 15 VG layouts.Then,the connection mode of the model,the change of angle of attack and the collection and processing of aerodynamic data were introduced.Finally,a wind tunnel experiment was performed to investigate the influence of height distribution of VG on aerodynamic characteristics of wing.The experimental results show that:(1)the height distribution of the three types of VGs could inhibit the stall flow and improve the aerodynamic performance of the wing;(2)“Triangular translation 3”and“Trapezoi-dal translation 2”were the best layouts,both of which could increase the maximum lift coefficient,delay stall,and significantly increase the lift-to-drag ratio at high angle of attack;(3)the influence of VG height and position factors on wing stall characteristics was analyzed by using the rough set theory,and the key position information of VG arrangement was provided.This study indicates that the variable height distribution is obviously better than the equal height distribution,which can provide ideas and refer-ences for the active control of variable height distribution of VGs.

Variable slip window technology in transition detection on pitching airfoil

In the pitching motion,the unsteady transition and relaminarization position plays an important role in the dynamic characteristics of the airfoil.In order to facilitate the computer to automatically and accurately calculate the position of the transition and relaminarization,a Variable Slip Window Technology(VSWT)suitable for airfoil dynamic data processing was developed using the S809 airfoil experimental data in this paper and two calculation strategies,i.e.,global strategy and single point strategy,were proposed:global strategy and single point strategy.The core of the VSWT is the selection of the window function h and the parameters setting in the h function.The effect of the VSWT was evaluated using the dimensionless pulse strength value(INB),which can be used to evaluate the signal characteristics,of the root mean square(RMS)value of the fluctuating pressure.It is found that:the h function characteristics have a significant influence on the VSWT.The suitable functions are Hn function constructed in this paper and step function.For the left boundary of the magnified area,the step function can obtain the largest INB value,but the robustness is not good.The H1 function(Gaussian-like function,n=1)can show higher robustness while ensuring a large INB value.The two computing strategies,which are single point strategy and global strategy,have their own advantages and disadvantages.The former strategy,that is the single point strategy,can achieve a higher INB value,but the RMS magnification at the feature position needs to be known in advance.Although the INB value obtained by the latter strategy,that is the global strategy,is slightly smaller than the calculation results of the former strategy,it is not necessary to know the RMS magnification at the feature position in advance.So the global strategy has better robustness.The experimental data of NACA0012 airfoil was used to further validate the developed VSWT in this paper,and the results show that the VSWT developed in this paper can still double the INB value of the transition/relaminarization position.The VSWT developed in this paper has certain practicability,which is convenient for the computer to automatically determine the transition/relaminarization characteristics.

Aerosol transportation in horizontal channels w让h gravitational effects

Aerosol particle deposition in a narrow horizontal channel with gravitational effects is studied in the laminar flow regime. A general explic让 solution for predicting the aerosol transportation in the channel is obtained theoretically. The characteristics of the asymmetric concentratio n profiles and their dependence on gravitational settling are analyzed for different settings of the diffusion and sedimentation parameters. Aerosol deposition on the channel walls, which is influenced by a combination of mechanisms, is also analyzed. The results are compared with those from simplified models obtained in previous studies in which only diffusion or gravity was considered. The analysis reveals that the literature results regarding this aspect may be simplified with the present solution under specific conditions.

Theoretical investigation on submicron particle penetration through circular tubes inside a porous medium

The analytical infinite series solution of submicron particle transport in a circular tube bounded by a porous wall,such as a pinhole,is determined under the slip velocity boundary condition,and the solution is verified by using the experimental data in the previous studies for the specific cases.The results show that particle penetration rate increases with the increase of the porous parameter,the axial pressure drop,and the pinhole radius,whereas it decreases with increasing the pinhole length.The penetration rate of nano-particles are more sensitive to the variation of these parameters.However,the differences between the penetrations of particles ranging from 0.3μm to 1μm are not evident because the diffusion becomes weak gradually in this size range.In addition,a further comparison is performed between the analytical solution and the existing studies,and approximate expressions are presented for accurate calculation of particle penetration rate through pinholes appearing in porous materials including filter devices and masks.

A conceptual design for deflection device in VTDP system

The effectiveness of the Vectored Thrust Ducted Propeller(VTDP)system is not high currently,especially the lateral force is not large enough.Thus,a conceptual design for a deflection device of a VTDP system was proposed to achieve effective hovering control.The magnitude of the lateral force that was applied to maintain balance while hovering was examined.A comparison between the experimental and numerical results for the 16H-1 was made to verify the numerical simulation approach.The deflection devices of the X-49 and the proposed design were analyzed using numerical simulations.The results indicated that a larger lateral force and lower power consumption were presented in the proposed design.The results of this article provide a new idea for the design of the VTDP system.