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Application of surrogate models to stability analysis and transition prediction in hypersonic flows
Advances in Aerodynamics volume 4, Article number: 33 (2022)
Abstract
To increase the efficiency and robustness of stabilitybased transition prediction in flow simulations, simplified methods are introduced to substitute direct stability analyses for rapid disturbance growth prediction. For lowspeed boundary layers, these methods are mainly established based on selfsimilar assumptions, which are not applicable to nonsimilar boundary layers in hypersonic flows. The objective of this article is to investigate the application of surrogate models to stability analysis of nonsimilar flows over blunt cones, focused on parameterization of boundarylayer (BL) profiles. Firstly, correlations between BL edge and profile parameters are analyzed, along with selfsimilar flow parameters and discrete points on BL profiles, which present four groups of BL characteristic parameters. Secondly, using these parameters as inputs, surrogate models are built for disturbance growth prediction over an MF1 blunt cone. Results show that, surrogate models using four BL edge parameters and a BL shape factor {U_{e}, T_{e}, ρ_{e}, η_{e}, H_{12}} for stability analysis can achieve comparable accuracy with those using 16 discrete BL profile parameters, which are more precise than those using merely selfsimilar parameters or BL edge parameters. Thirdly, the established surrogate models are validated by stability analysis and transition prediction over the MF1 blunt cone in flight experiments at the instants of t = 17 s ~ 22 s. Compared with direct linear stability analyses, the mean relative error of predicted disturbance growth rates by surrogate models is 8.0% and the maximum relative error of N factor envelopes is 6.6%, which indicates feasible applications of surrogate models to stability analysis and transition prediction of nonsimilar boundary layers in hypersonic flows.
Introduction
Laminartoturbulent transition of boundary layers has attracted growing concern in the aerodynamic shape design of aircrafts. For example, naturallaminarflow design for transonic/supersonic transports needs to delay transition onset to reduce friction drag, and special thermal protection measures are required for hypersonic vehicles near transition regions, where heat fluxes increase rapidly. NASA’s CFD Vision 2030 [1] regards physicsbased and predictive modeling including boundarylayer transition as a basic capability for future CFD techniques. Therefore, accurate prediction and modeling of boundarylayer transition has become a critical requirement in aerodynamic design of nextgeneration aircrafts.
In order to take into account flow transition in CFD simulations, many transition prediction methods have been proposed. These methods include: 1) stabilitytheorybased e^{N} method, which is a physicsbased method modeling the evolution of perturbations inside boundary layers, e.g. the local linear stability theory (LST) [2,3,4,5], Biglobal LST [6], parabolized stability equations (PSE) method [7, 8] and direct numerical simulation method [9, 10]. 2) Transition criteria, which utilize empirical criteria derived from theoretical and experimental study, e.g. ArnalHabiballahDelcourt (AHD) criteria [11], Gleyzes criteria [12] and C1 criteria [13]. 3) Transition transport models, which implement empirical transition criteria into transport equations during flow simulation, e.g. γRe_{θ} model [14, 15], kωγ model [16, 17] and amplification factor transport (AFT) model [18]. The e^{N} method was firstly proposed in 1956 [2] and then modified for 2D, 3D, incompressible and compressible boundary layers [19]. It has been coupled with RANS simulations and applied in boundarylayer transition prediction for airfoil/wing/wingbody configurations [20,21,22,23,24]. Among the existing transition prediction methods for hypersonic boundary layers, the e^{N} method is also one of the most widely applied and effective methods, having been calibrated by various wind tunnel and flight experiments.
While successful engineering applications of the e^{N} method have been devised to model the complex physics, there is still a great need to improve the efficiency and robustness of these applications [25]. For example, a common way of using the e^{N} method is to couple a LST solver with a RANS solver in an iterative manner [22]. The RANS solver provides boundary layer information for stability analysis. The LST solver computes growth rates and amplification factors (N factors) of perturbations at different frequencies and wavenumbers, and locates the transition onset where the N factors reach a threshold value. Coupling of the LST solver and RANS solver is implemented only after the boundary layer flow attains a level of maturity. The predicted transition location by LST is then returned to RANS simulations to substitute a former one and the turbulence models are updated accordingly. In this process, highaccuracy boundary layer profiles are needed for stability analysis, including velocity and temperature profiles, as well as their first and second derivatives [22]. This demands highresolution flow simulations inside boundary layers, accompanied with heavy computational costs. In e^{N} transition prediction, a number of unstable disturbances at different frequencies and wavenumbers have to be evaluated, which also increases computation efforts. Besides, the calculation of amplification factors N is a nonlocal operation, which makes it quite difficult to implement the e^{N} method in a general CFD code. To convert N factors calculations to a local process and simplify the application of e^{N} method, the AFT model [17] is proposed, implementing a relation between N factor envelope slope and boundarylayer shape factor into transport equations. Whereas, the AFT method is mainly appropriate for lowspeed flows, and further research is still necessary to extend this method for disturbance growth prediction in hypersonic flows.
As a substitute of solving linear stability equations, surrogate models are introduced to simplify the disturbance growth calculation. The application of surrogate models in linear stability analysis was firstly presented by Fuller et al. [26] in 1997, who trained a neural network to identify the growth rates at different streamwise locations in a jet flowfiled. The predicted disturbance growth rate is very accurate with less than 2% error interpolating within the training set. In 2002, Crouch et al. [27] used neural networks to predict the growth rate of TollmienSchlichting (TS) waves and stationary crossflow (CF) instabilities on swept wings. Rajnarayan and Sturdza [25] introduced singular value decomposition method to describe the boundarylayer velocity and temperature profiles in the modeling of separate TS and CF modes for supersonic flows in 2013. Zafar and Xiao et al. [28] built a convolutional neural network for lowspeed airfoils and found the relationship between boundary layer profiles and boundary layer shape factor. For hypersonic boundary layers, Pinna F et al. [29] selected a Gaussian basis function and used a compact support radial basis function method to predict the disturbance growth on sharp cones in 2018. Their training set was generated by stability analysis on selfsimilar boundary layer profiles. Danvin et al. [30, 31] investigated the accuracy of disturbance growth prediction by surrogate models on sharp cones using selfsimilar profiles and highaccuracy CFD profiles, respectively. Results show that surrogate models built based on selfsimilar profiles are more accurate than those based on CFD profiles. Whereas, Pinna and Danvin’s research dealt primarily with sharp cones, where entropy layers or nonsimilar effects hardly exist. In 2020, Paredes et al. [32] used full CFD profiles with 61 equalspaced points inside the boundary layer to build a convolutional neural network for stability analysis on a blunt cone. Their model can precisely predict the disturbance growth during the ascent phase of the HIFiRE1 flight experiment. Moreover, it even outperforms direct linear stability calculations for some underresolved basic states. Nie et al. [33] provided a surrogatebased stability analysis method for oblique first modes and Mack modes in selfsimilar compressible boundary layers at Mach 0 ~ 6.
Compared with directly solving stability equations in transition prediction, using surrogatebased method is more efficient and robust. It can reduce the requirement for boundary layer resolution in CFD simulations [32], as it needs merely several characteristic BL parameters, and has the potential to be transformed into local formulations for transition transport modeling. Compared with other simplified e^{N} methods like e^{N}envelopefitting method and e^{N}database method, a surrogatebased method can predict growth rates of separate modes at different frequencies and wavenumbers and achieve comparable accuracy to a LST method.
In general, there are two ways of parameterizing BL profiles when establishing surrogate models for disturbance growth prediction in hypersonic boundary layers. One is to describe the boundary layer using selfsimilar flow parameters, such as boundarylayeredge Mach number M_{e}, temperature T_{e} and Hartree pressure gradient parameter β_{H}. It is suitable for sharp cones without nonsimilar effects, but generates some deviation for blunt cones. The other is to describe the boundary layer using discrete points on the BL profiles extracted from highaccuracy CFD results. It can deal with nonsimilar boundary layers and predict the disturbance growth on blunt cones with high accuracy. Whereas, this method faces the same problem as a direct linear stability analysis method, which needs highresolution boundary layer profiles. It is also difficult to be further transformed into local formulations, which is necessary in transition transport modeling.
The objective of this article is to build surrogate models for rapid disturbance growth prediction in stability analysis and transition prediction of hypersonic boundary layers. Based on previous studies, we aim to find a proper method to describe boundary layer profiles, which is expected to be not only applicable to nonsimilar boundary layers near the nose of a blunt cone, but also capable of being transformed into local formulations for transition transport modeling. Instead of directly using discrete points on the boundary layer profiles by Danvin and Paredes, we have conducted correlation analyses between boundary layer flow parameters and profile parameters and found a group of characteristic parameters for boundary layer description.
This article continues in Section 3 to introduce three methods for boundary layer profile description in surrogate model training, including selfsimilar flow solutions, discrete point on BL profiles and correlated boundarylayer flow parameters. Section 4 presents a surrogatebased stability analysis method for hypersonic boundary layers over blunt cones. Freestream conditions are selected according to the MF1 flight experiments. Radial basis function (RBF) models are established based on highaccuracy CFD solutions, with those characteristic BL parameters as inputs. The predicted disturbance growth rates and amplification factors for MF1 experiments at the instants of t = 17 s ~ 22 s by RBF models are compared with those by LST to validate the boundary layer description methods and the surrogatebased stability analysis method.
Theory
Linear stability theory and e^{N} method
To predict the transition location, linear stability theory introduces a small disturbance inside the boundary layer based on parallel flow assumption. Initial disturbances at different frequencies are located on neutral curves, where those disturbances begin to amplify. By solving linear stability equations, the disturbance growth rates are computed at each streamwise station and then integrated into an amplification factor N. The transition onset predicted by the e^{N} method is located where the amplification factor N reaches a precalibrated threshold N_{cr}. The small disturbances in linear stability theory are defined as
where α_{r} is streamwise wavenumber, α_{i} is streamwise growth rate, ω is circular frequency, x and y are the streamwise and normal coordinate. To compute the neutral curves or disturbance growth rates, an eigenvalue problem is solved for a given boundary layer as
where neutral curves are located with local Reynolds number Re_{l} and growth rate α_{i} = 0, and disturbance growth is calculated with local Reynolds number Re_{l} at certain circular frequency ω. Once the disturbance growth at different frequencies is solved, amplification factors are computed along the streamwise location by
where x_{0} is the location that disturbances at a certain frequency f begin to amplify. According to the transition mechanisms and disturbance parameters such as wavenumber, frequency and phase velocity, unstable disturbances can be classified as first modes (TollmienSchlichting waves), second modes (Mack modes) and others. The surrogatebased stability analysis method in this article is not restricted to a specific kind of unstable modes, but applies to disturbances within a range of frequencies and wavenumbers.
Radial basis function interpolation [34]
Surrogate models (or metamodels) such as kriging models, support vector machine models, neural networks and so on, have been widely used in aerodynamic design [35,36,37]. The RBF (radial basis function) model is one of the simplest datadriven surrogate models. Its interpolation coefficients are calculated for a given sample set based on the distances among sample points. Assuming there are n sample points x_{1}, x_{2}, …, x_{n} whose response values are f_{1}, f_{2}, …, f_{n}, a system of linear equations can be obtained as
where ϕ is the basis function, λ is the interpolation coefficient and x_{i}  x_{j} is the Euclidean distance between the ith and jth sample points. Although the RBF model is a combination of linear equations, it can well fit complex nonlinear problems with sufficient sample points. During model training, the correlation matrix sometimes meets an illconditioning problem, resulting from a large condition number and leading to numerical error. To avoid this problem, a regularization method is introduced by adding small values on the diagonal of the Matrix to reduce its condition number. To predict the response value of an unknown point, the distances of this point with the precalculated sample points are computed as inputs. The response value f_{p} at a new point x_{p} is predicted as
The basis function in this article is a thinplate spline function, which is ϕ(r) = r^{2} log r.
Boundary layer description methods
As mentioned above, solving the eigenvalue problem for disturbance growth rate requires detailed BL information, including velocity and temperature profiles as well as their derivatives. According to the convergence study by Fischer et al. [22], at least 60 or more points are needed inside the boundary layer to ensure the accuracy of stability analysis. To represent the full BL profile, a few characteristic parameters are usually introduced. For example, a single parameter of BL shape factor H_{12} can be used to characterize a lowspeed boundary layer on a flat plate or airfoil, as the BL flow shows selfsimilar characteristics. When it comes to hypersonic boundary layers on blunt cones, the blunt nose results in offbody shock waves and entropy layers, which could bring nonsimilar effects to BL profiles. To find a proper BL description method for such nonsimilar boundary layers, three kinds of BL characteristic parameters are investigated, which are selfsimilar flow parameters, discrete points on BL profile and correlated BL flow parameters.
Selfsimilar flow parameters
Solution of selfsimilar profiles on a sharp cone at zero angle of attack refers to [38]. Firstly, the boundary layer equations of a sharp cone in polar coordinate are given as
where r and θ are polar coordinates as shown in Fig. 1, h is enthalpy, U and V are streamwise and normal velocities, ρ is density, μ is dynamic viscosity, and Pr is Prandtl number. Applying a transformation of \(\eta =r\left(\theta {\theta}_0\right)/\sqrt{r}\) and \(\overline{V}=V\cdot \sqrt{r}\), where θ_{0} is the cone angle, the boundary layer equations are reduced to a system of total differential equations as
To resolve the “2ρU” terms in the continuity equation, an additional transformation of \(\tilde{\eta}=\sqrt{3}\eta\) and \(\overline{V}=\sqrt{3}\left(\tilde{V}2/3\cdot \tilde{\eta}U\right)\) is introduced. After this, the boundary layer equations of a sharp cone are converted to the same formulation as that of a flat plate. As a result, the similarity solutions of sharpcone boundary layer equations can be obtained as on a flat plate, which refers to [39]. Considering the pressure gradient over cone surface, solving the similar equations requires only boundarylayer edge Mach number M_{e}, temperature T_{e} and Hartree parameter β_{H} (\({\beta}_H=\frac{2m}{m+1},m=\frac{\xi }{u_e}\frac{{d u}_e}{d\xi},\xi =\int_0^{x_s}\limits{\rho}_e{u}_e{\mu}_e{d x}_s\)). Therefore, according to similar solutions on sharp cones, a parameter set {M_{e}, T_{e}, β_{H}} (marked as A) is used as the inputs of MODEL A for surrogate model training in the next section.
Discrete boundarylayer profile parameters
It has been proved by [32] that using selfsimilar profiles generates some deviation from the CFD profiles on blunt cones. To consider the nonsimilar effects, a simple method is to use discrete points inside boundary layers to represent BL profiles. As an example, the velocity and temperature value as well as their derivatives are extracted at four equalspaced locations on the BL profiles in Fig. 2. This parameter set {(u/u_{e})_{0.2δ}, (u/u_{e})_{0.4δ}, (u/u_{e})_{0.6δ}, (u/u_{e})_{0.8δ}, (u/u_{e})’_{0.2δ}, (u/u_{e})’_{0.4δ}, (u/u_{e})’_{0.6δ}, (u/u_{e})’_{0.8δ}, (t/t_{e})_{0.2δ}, (t/t_{e})_{0.4δ}, (t/t_{e})_{0.6δ}, (t/t_{e})_{0.8δ}, (t/t_{e})’_{0.2δ}, (t/t_{e})’_{0.4δ}, (t/t_{e})’_{0.6δ}, (t/t_{e})’_{0.8δ}} (marked as B) will be used as the inputs of MODEL B for surrogate model training in the next section.
Correlated boundarylayer flow parameters
According to selfsimilar boundary layer solutions, BL edge flow parameters are highly correlated to BL profiles. Although using selfsimilar parameters is not enough to represent nonsimilar boundary layers on blunt cones, additional BL edge parameters can be included to increase accuracy. Those nonsimilar effects originate from offbody shock waves and entropy layers near the blunt cone nose, which are mainly influenced by freestream Mach number M_{∞} and nose radius of cone [40]. Therefore, we conducted correlation analyses between BL edge and profile parameters based on CFD simulations over blunt cone at different freestream Mach numbers and different nose radiuses. In this study, the positions of BL edge are defined between the maximumenthalpy point and farfield, where local enthalpy reaches the average value of maximumenthalpy and freestream enthalpy, as depicted in Fig. 3.
Correlation of BL parameters at different freestream Mach numbers M _{∞}
With fixed nose radius of 5 mm, boundary layer flows of blunt cones are computed at freestream Mach numbers from M_{∞} = 4 to M_{∞} = 6. The half cone angle is 7°, freestream Mach number is selected at M_{∞} = 4, 4.2, 4.4, …, 6.0, unit Reynolds number is Re_{∞} = 2 × 10^{7}/m and angle of attack is 0°. Referring to [40], the entropy layer edge is defined at the location where local entropy production reaches 0.01 times the maximum entropy production at stagnation point. The Mach number contour and entropy layer edge distributions at different Mach numbers are presented in Fig. 4. It can be seen that the location of entropy layer edge hardly moves with the increase of Mach number, but the absolute value of entropy production inside the entropy layer is larger. The pressure distribution at BL edge is also given in Fig. 5, which shows that the absolute value of pressure increases with the Mach number, but its relative distribution varies only a little. Correlation analyses are conducted between BL flow parameters using BL data at s = 0.2 m ~ 1.0 m for all the freestream conditions. Using the extracted characteristic BL parameters and BL profile parameters, the square Pearson correlation coefficients are computed by
where x_{i} is the characteristic boundary layer parameter of the ith profile, y_{i} is the boundary layer profile parameter of the ith profile, \(\overline{x}\) and \(\overline{y}\) are the averaged values, respectively. It is shown in Tables 1 and 2 that velocity profiles are most related with s_{e}, T_{e}, η_{e}, H_{12} and β_{H}, while temperature profiles are correlated with ρ_{e} and u_{e}. Furthermore, the entropy production s_{e} can be computed using T_{e} and ρ_{e}. The shape factor H_{12} is usually estimated by Mach number M_{e} and Hartree pressure gradient factor β_{H}. Therefore, the parameter set {U_{e}, T_{e}, ρ_{e}, η_{e}, β_{H}} (marked as C) and {U_{e}, T_{e}, ρ_{e}, η_{e}, H_{12}} (marked as D) are expected to be appropriate for BL profiles over blunt cones at different Mach numbers. Note that parameter set C only uses BL edge parameters, while set D includes an integral BL shape factor. These two parameter sets will be validated to see if they are suitable for blunt cones with different nose radiuses.
Correlation of BL parameters at different nose radiuses R _{N}
With fixed freestream Mach number at M_{∞} = 5, boundary layer flows are computed for blunt cones with nose radiuses from r_{n} = 2 mm to r_{n} = 8 mm. The half cone angle is 7°, freestream Mach number is M_{∞} = 5.0, unit Reynolds number is Re_{u} = 2 × 10^{7}/m and angle of attack is 0°. The Mach number contour and entropy layer edge distributions at different nose radiuses are presented in Fig. 6. It can be seen that the absolute value of entropy production changes very little with nose radius, but the location of entropy layer edge moves outward. The pressure distribution at BL edge is also given in Fig. 7. Pressures experience the effects of upstream bluntness right up to a surface length of about s = 0.5, beyond which point they settle down to a uniform value. In the bluntness region, the absolute pressure values diminish with bluntness past the minimum value. Correlation analyses are conducted between BL parameters using BL data at s = 0.2 m ~ 1.0 m for all the nose radiuses. The computed square Pearson correlation coefficients are listed in Tables 3 and 4. It can be seen that the relationship between velocity profiles and H_{12} is still close, so is that between temperature profiles and ρ_{e} and u_{e}. But the correlation factor between velocity profiles and other parameters becomes much lower, which can influence the effects of parameter set C. We will further use the parameter sets C and D as inputs in training surrogate models in the next section to see which of them is more accurate.
Brief summary of BL characteristic parameters
In Section 3, three methods are discussed for BL profile description, including using selfsimilar flow parameters, discrete BL profile parameters and correlated BL flow parameters. As a result, four sets of parameters (marked as A, B, C and D) are derived as inputs in establishing surrogate models for stability analysis over blunt cones, which are summarized in Table 5. To further study the accuracy of the four parameter sets A ~ D in stability analysis modeling, surrogate models A ~ D using those parameters as inputs will be investigated in Section 4.
Surrogatebased stability analysis on MF1 blunt cone at flight experiments [41]
Based on four sets of characteristic parameters, surrogate models are established for stability analysis over MF1 blunt cone at flight experiments [41]. The main purpose of using surrogate models is to substitute linear stability solvers for rapid disturbancegrowth prediction. Apart from better efficiency and robustness, the surrogatebased method also removes the necessity of highresolution boundary layer solutions, which can reduce the computational cost of CFD simulations and has the potential to be further implemented in transition transport modeling.
The process of building surrogate models in this article is described as follows. Firstly, a database is generated by performing CFD simulations and stability analyses on MF1 blunt cones at different freestream conditions. Secondly, characteristic flow parameters are extracted as input of surrogate models, along with the disturbance parameters. Thirdly, the complete sample set is partitioned into smaller sample sets and RBF models are trained for each subset. Finally, the trained RBF models are used in stability analysis and transition prediction. The flow charts of disturbance growth prediction by direct linear stability analysis and by surrogate models are depicted in Fig. 8, respectively.
Database generation
Considering the nonsimilarity of hypersonic boundary layers on blunt cones, it’s difficult to establish a universal surrogate model like in lowspeed boundary layers over flat plates or airfoils. The objective of using surrogate models is thus to satisfy the requirement of disturbance growth prediction in transition prediction within a particular range of configurations and freestream conditions. In this article, the MF1 blunt cone [41] during ascent phase at flight experiments is chosen to validate the surrogatebased stability analysis method. Its half cone angle is 7° and nose radius is 5 mm. Freestream conditions of the MF1 cone at flight experiments are listed in Table 6.
In order to satisfy the above condition, flow parameters for calculating training samples are selected at Mach numbers M_{∞} = 4.3: 0.2: 5.5, unit Reynolds number Re_{∞} = 1.4 × 10^{7}: 0.3 × 10^{7}: 3.5 × 10^{7} and wall temperatures T_{w} = 500: 25: 600, as listed in Table 7, which add up to 280 base flows. The computational states of the MF1 flight experiment can be interpolated from the training samples, as shown in Fig. 9. The base flows are computed using a Laminar NS solver using a second order upwind scheme. The BL profiles are extracted for linear stability analysis. Characteristic BL profile and edge parameters are also extracted according to the four groups of sample sets A ~ D.
Based on the extracted BL information, a linear stability solver is adopted to compute the growth rates α_{i} of perturbations at nondimensional circular frequency of ω = 0 ~ 1.0 at all the streamwise locations. An example of the computed growth rate contour at different frequencies is demonstrated in Fig. 10. Although the physical frequency of unstable modes varies a lot at different streamwise directions, it is observed that the neutral curves of all the unstable modes in the training sample set are located within the range of nondimensional circular frequency of 0.38 < ω < 0.72 and local Reynolds number of Re_{l} > 2000. To cater for possible disturbance growth rates remaining below zero, stability analysis results are extracted at 31 circular frequencies from ω = 0.35 to ω = 0.75 and 31 streamwise locations at Re_{l} > 1800.
Finally, growth rates of disturbance at the 31 frequencies and 31 streamwise locations for 280 freestream conditions are used as response values in the training samples, together with the A, B, C, D four sets of boundary layer parameters and disturbance parameters as input values, which add up to 269,080 sample points.
Surrogate model training
RBF models with thinplate spline basis function are established to predict disturbance growth. Considering the difficulties of model training brought about by a large number of sample points, a sample partition method is utilized to classify the complete sample set into a series of sub sample sets. Thus, models are trained within each of the subsamplesets. When used for prediction, the validation samples also need to search for their corresponding subsampleset and surrogate model. Based on this idea, the database with 269,080 sample points is partitioned into 25 subsets. In the 25 partitions (5 × 5), five are selected on the basis of nondimensional frequency, and the other five are based on local Reynolds number criteria (as shown in Fig. 11). The number of sample points within each of the sample set is near 12,000. Using the four sets of characteristic parameters A, B, C and D, RBF models are trained (also marked as A, B, C and D) and their prediction accuracies are compared in Section 4.3.
Comparison of prediction accuracy of RBF models using different input parameters
To assess the accuracy of the four sets of surrogate models, validated samples are selected by stability analysis on MF1 blunt cones at the instants of t = 17 s ~ 22 s during ascent of flight experiments. Predicted growth rates are compared with those computed by linear stability analysis. Firstly, the computed disturbance growth rate at different frequencies at the moment of t = 17 s ~ 22 s by those RBF models and by LST are compared in Fig. 12. It can be seen that the predicted values by RBF model B and D agree well with those by LST, while the results of RBF A and C show some deviations. The relative errors of all validation samples using the four RBF models have been gathered and listed in Table 8. It is demonstrated that RBF models using 16 discrete BL profile parameters (models B) and using four BL edge parameters and a shape factor (models D) are the most accurate. They are obviously more precise than those using selfsimilar flow parameters (models A) and using merely BL edge parameters (models C). As expected, the mere use of selfsimilar profiles or BL edge parameter to describe BL profiles overlooks the nonsimilar effects in blunt cone boundary layers, which can lead to deviation from realistic BL profiles. The N factor envelopes computed by LST and RBF models are also compared at different streamwise locations, as shown in Fig. 13. It can be seen that, regardless of the deviations in predicted growth rates, the RBF models using selfsimilar parameters and BL edge parameters can still provide reasonable prediction for N factors. While RBF models using discrete profile parameters and those using BL edge parameters and shape factor are more accurate, especially at t = 17 s and t = 21 s.
Validation of RBF models using BL characteristic parameters in stability analysis and transition prediction
It has been shown in Section 4.3 that RBF models using merely four BL edge parameters and a shape factor can be as accurate as those using 16 discrete BL profile parameters. In view of the potential that BL edge parameters and shape factor can be converted into local variables and used for transition transport modeling, we would like to further investigate the accuracy of RBF models with those parameters in boundary layer stability analysis and transition prediction.
The contours and distributions of predicted disturbance growth rates by RBF models D (using four BL edge parameters and a shape factor as inputs) are compared with those by LST in Fig. 14. As can be seen from the figure, the predicted growth rate of RBF models D at instants from t = 17 s to t = 22 s agrees well with those by LST, which means that the established models D are applicable to nonsimilar boundary layers on hypersonic blunt cones. Computational costs of the two methods are also given in Table 9. To predict growth rates of a group of disturbances at different frequencies and streamwise locations (3600 samples in total), RBF models take only 13.38 s, while the LST method requires about 877.32 s. The reason for saving time cost by surrogatebased method is that it uses precalculated results to interpolate the growth rates, while the direct linear stability analysis method needs to solve the stability equations iteratively.
The amplification factors of disturbances on MF1 blunt cones are also computed by RBF D models and the LST method, respectively. As shown in Fig. 15, the N factors at different frequencies and their envelopes computed by RBF D models are very close to those by the LST method. The maximum relative error of N factor envelope is 6.6%, demonstrating the accuracy of disturbance growth prediction using RBF models with four BL edge parameters and a shape factor for nonsimilar blunt cone boundary layers. It also shows that the established surrogate models are a good substitute for directly solving stability equations in e^{N} transition prediction.
Conclusion
To improve the efficiency and robustness of e^{N} transition prediction, we have investigated application of surrogatebased stability analysis for nonsimilar boundary layers, focused on input parameters for boundarylayer (BL) profile description. Four sets of BL parameters are proposed as input parameters for establishing surrogate models. The accuracies of these models are validated through MF1 flight experiments and compared with direct linear stability analysis. Some conclusions are obtained as follows.

(1)
The proposed surrogatebased method can substitute direct stability analysis for rapid disturbance growth prediction on hypersonic blunt cones. It can increase the efficiency of e^{N} transition prediction, which gets rid of the necessity of highresolution BL profiles, initial eigenvalue search and iterative solution of stability equations. Because the surrogate models merely use BL edge parameters and shape factor, they also have the potential to be transformed into local formulations and implemented in transition transport models for general CFD applications.

(2)
Correlation analyses demonstrate that BL edge parameters and shape factor are highly correlated with BL profiles. Surrogate models using merely four BL edge parameters and a shape factor {U_{e}, T_{e}, ρ_{e}, η_{e}, H_{12}} for stability analysis and transition prediction can achieve comparable accurate results with those using 16 discrete boundary layer profile parameters.

(3)
For MF1 blunt cone at flight experiments, the mean relative error of predicted disturbance growth rates by established surrogate models is 8.0% and the maximum relative error of calculated N factor envelopes is less than 6.6%, which validates the applicability of surrogatebased methods using merely BL edge parameters and shape factor to stability analysis and transition prediction of nonsimilar boundary layers over blunt cones.
Availability of data and materials
All data and materials are available from the corresponding author upon reasonable request.
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This work is supported by the National Numerical Wind Tunnel Project (No. NNW2018ZT1A03) and the National Natural Science Foundation of China (No. 12072285 and No. 11972305).
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Nie, H., Song, W., Han, Z. et al. Application of surrogate models to stability analysis and transition prediction in hypersonic flows. Adv. Aerodyn. 4, 33 (2022). https://doi.org/10.1186/s42774022001202
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DOI: https://doi.org/10.1186/s42774022001202
Keywords
 Surrogate models
 Stability analysis
 Transition prediction
 Hypersonic flows
 Blunt cone