Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a gas turbine wheel shape optimization design method based on Bayesian optimization and GCN (graph convolution neural network), which adopts Bezier curves to parameterize wheel types, considers the temperature gradient load from high-temperature high-pressure fuel gas, and fits the actual working condition of the wheel, and has the characteristics of short design period, wide optimization range, effective and reliable optimization design result, and important engineering application benefit and popularization value.
The invention is realized by adopting the following technical scheme:
a shape optimization design method of a turbine wheel disc of a gas turbine based on Bayesian optimization and GCN comprises the following steps:
Firstly, carrying out parameterization modeling on a turbine wheel disc geometry of a gas turbine to be optimized by adopting a Bezier curve, determining geometric parameters serving as design variables, determining a value range of the geometric parameters, and constructing an optimization space of the geometric design variables of the wheel disc;
Sampling design variables in a design domain based on an optimization space of the geometric design variables of the wheel disc, establishing a geometric model of the wheel disc by using sample data, respectively carrying out finite element calculation and analysis according to the geometric model to obtain maximum radial deformation, maximum stress value and quality data of the wheel disc, and constructing a geometric structure optimization design database of the wheel disc;
respectively carrying out normalization operation on data in the wheel disc geometric structure optimization design database, and dividing the data into a training set and a verification set according to the proportion of 7:3 after random scrambling, wherein the training set and the verification set are used as a data set of a graph convolution neural network;
step four, constructing a graph convolution neural network, and training by using the data set in the step three to obtain a wheel disc prediction model distributed from geometric parameters to a displacement field and a stress field;
and fifthly, performing target optimization on the wheel disc by adopting a Bayesian optimization method and combining a trained wheel disc prediction model, and performing automatic optimization on a wheel disc combined structure to obtain a final optimal design scheme of the wheel disc geometric structure.
The invention is further improved in that the first step specifically comprises:
the Bezier curve is adopted to parameterize the wheel disc molded line, and the expression is as follows:
Wherein P (t) is a point on a Bezier curve, P i is a Bezier curve control point, B i,n (t) is a Bernstein base function, t is a curve point generation parameter, n is a Bezier curve order, and t is continuously changed in a range of [0,1] to determine a corresponding Bezier curve after the curve order n and the control point P i are determined;
And marking the transverse and longitudinal sitting of the ith control point as [ x i,yi ], taking the coordinates of all the control points as geometric design variables, and constructing an optimization space of the geometric design variables of the wheel disc, wherein the value range of the geometric design variables is +/-20% of the initial value.
The invention further improves that the turbine wheel disc part of the gas turbine is of an axisymmetric structure, and the two-dimensional optimization design is carried out aiming at the wheel surface molded line, namely, when the geometric structure of the wheel disc is optimized, the wheel surface molded line is designed, and the displacement and stress distribution conditions of the wheel surface molded line are considered.
The invention is further improved in that the Bezier curves with different orders are used for parameterizing different parts of the wheel disc.
The invention is further improved in that the second step specifically comprises:
Sampling in an optimization space formed by coordinates of each intermediate control point of a Bezier curve by using a Latin hypercube sampling method to obtain a wheel disc sample set S, establishing the Bezier curve for each sample in the sample set S to obtain a wheel disc geometric model, calling finite element calculation software to perform grid division and numerical calculation to obtain a wheel disc displacement field f 1 and a stress field f 2, obtaining the maximum radial deformation delta u x,max and the maximum stress value sigma max of the wheel disc, simultaneously calculating the corresponding wheel disc mass m by combining wheel disc material parameters, deriving the wheel disc grid node coordinates C= [ X, y ], and jointly forming a database [ X ] = { S, C, f 1,f2,Δux,max,σmax, m } of the optimization design of the wheel disc geometric structure by using design variables and target parameters.
The invention is further improved in that the third step specifically comprises:
normalizing the data set according to the formula:
Wherein [ X j ] is the j-th dataset in the database, min and Max respectively represent the maximum value and the minimum value of each dimension data in the corresponding dataset, epsilon=1×10 -6 is a small amount;
generating random numbers will normalize the data set Randomly sorting and dividing the training sets into training sets according to the proportion of 7:3And a verification set
The invention is further improved in that the step four specifically comprises:
Construction of displacement field and stress field prediction model GNet based on graph convolution neural network, specifically, GNet is composed of an input layer, a graph convolution layer and an output layer, wherein the input layer is composed of a full-connection layer and an activation function, the graph convolution layer is composed of a 6-layer graph convolution operator and an activation function, the output layer is a 1-layer graph convolution operator, GNet is input with a sample set S and wheel disc profile grid node coordinates C, and the predicted displacement field of the wheel disc profile is output Stress fieldThe network mapping relation is as follows:
in the formula, The method comprises the steps of obtaining a predicted displacement field or stress field of a wheel disc molded surface, wherein F is a graph convolution map, S is single sample data comprising the abscissa of a Bezier curve control point, C is the grid node coordinate of the single sample, and Θ is a parameter to be learned of a network;
utilizing a data set Training GNet, utilizing a datasetVerification is performed during the training process.
The invention further improves the method, based on the obtained predicted stress field, for monitoring and judging the stress concentration phenomenon of the key part of the wheel disc structure, and specifically comprises the following steps:
Wherein, sigma loc,max is the local stress maximum value of the key part of the structure, sigma loc,m is the local stress average value, sigma th is the stress threshold value for stress concentration monitoring, and K is the local structure stress concentration factor;
Firstly, determining a stress threshold sigma th based on turbine wheel disc materials of a gas turbine and working conditions of high temperature, high temperature gradient and high rotating speed, secondly, monitoring stress values of key parts of a wheel disc structure based on an obtained predicted stress field, judging the stress concentration degree if the maximum stress value sigma loc,max exceeds sigma th, and finally, calculating a local structure stress concentration factor K by sigma loc,max and sigma loc,m to represent the stress concentration degree of the key parts.
The invention is further improved in that the fifth step specifically comprises:
And carrying out loop iteration on the wheel disc molded line design by combining the high-precision wheel disc displacement field stress field prediction model obtained by training through a Bayesian optimization method, wherein the Bayesian optimization carries out automatic optimization on the wheel disc molded line data by taking a priori function and an acquisition function as cores, the prediction model carries out displacement field stress field prediction according to the newly generated wheel disc molded line to obtain displacement field distribution, maximum radial deformation and stress field distribution and maximum stress, simultaneously calculates the mass of the corresponding wheel disc, and continuously carries out recommendation and evaluation under the target requirement that the maximum radial deformation is smaller than the allowable radial deformation, the maximum stress is smaller than the allowable stress and the mass is minimum, thereby finally obtaining the optimal design scheme of the wheel disc structure.
The invention is further improved in that the prior function in Bayesian optimization adopts widely used Gaussian process regression, the acquisition function adopts an improved STABLE form STABLE-EI of expected increment EI widely used in standard Bayesian optimization, and the improved STABLE form STABLE-EI has good robustness, so that the Bayesian optimization process is easier to obtain a global optimal solution;
Wherein ,vt=σt(x,ΣX);zt=[mt(x,∑X)-ωσt,a(x,∑x)-f(x+)]/vt;Φ(z) is a standard normal cumulative distribution function, phi (z) is a standard normal probability density function, m (x) is a mean function, omega is a weight for penalizing the midpoint of the unstable region, f (x) is a function from gaussian process regression, x +=arg maxf(xi.
Compared with the prior art, the invention has at least the following beneficial technical effects:
The invention provides a wheel disc structure optimization design method based on Bayesian optimization and graph convolution neural network by integrating a plurality of prior art and carrying out improvement and innovation on the gas turbine wheel disc structure optimization design method. The parameterized model, sampling calculation, database establishment, high-precision wheel disc displacement field stress field prediction model GNet obtained by training and automatic optimization by combining Bayesian optimization with the prediction model can be realized through one script in the whole process, no manual participation or intervention is needed in the middle, and the method is simple and efficient; compared with the traditional wheel disc optimization method, the method provided by the invention improves the whole flow of the wheel disc structural design, and has the advantages of stronger geometric structure expression capability, larger optimization exploration breadth and shorter optimization iteration period.
Furthermore, the invention adopts the Bezier curve to carry out subsection parameterization on the wheel disc molded line, the description capability of the required optimized curve is strong and the curve is continuous and smooth on the premise of ensuring the geometric parameters of the key part of the wheel disc, and compared with the traditional parameterization method, the invention can generate complex molded lines more conveniently and realize the fine description of the geometric shape of the wheel disc.
Furthermore, the Latin hypercube sampling method is adopted to obtain the sample set, the sampled sample set has good representativeness and descriptive property on the optimization space, and a database which is enough to support and train out a high-precision displacement field stress field prediction model can be obtained under the condition of the minimum sample number.
Furthermore, the invention constructs the stress field prediction model of the wheel disc displacement field based on the graph convolutional neural network, can realize the field prediction of any irregular grid, obviously is difficult to realize the regular division of the finite element grid for the physical model with complex molded lines such as the wheel disc, and fundamentally solves the problem that the convolutional neural network can only process the regular grid.
Further, the invention provides a method for monitoring and judging stress concentration phenomena at key parts of a structure, corresponding stress thresholds are determined based on material parameters and working conditions of a turbine wheel disc of a gas turbine, firstly, local stress concentration conditions are monitored, if the stress concentration conditions exceed the thresholds, local stress concentration degrees are represented by calculating stress concentration factors, and the local stress concentration conditions at the key parts are considered while the geometric structure is optimized.
Furthermore, the invention adopts the Bayesian optimization method which takes the Gaussian process regression as the prior function and the STABLE-EI as the acquisition function to carry out automatic optimization, wherein the proxy model constructed by the Gaussian process regression has high precision, the STABLE-EI can converge the function optimization to a STABLE peak value, and the turbine wheel disc geometry of the gas turbine meeting the design target can be rapidly found.
Furthermore, a Bayesian optimization method with widely verified performance in parameter optimization is combined with a high-precision wheel disc displacement field stress field prediction model constructed based on a graph convolution neural network, and the optimal design of the wheel disc structure with the lightest mass under the conditions of meeting the maximum radial deformation and allowable stress is realized in an optimization space.
In conclusion, the invention has important engineering significance and wide application prospect.
Detailed Description
The invention is further described below with reference to the accompanying drawings.
Referring to fig. 1, the method for optimizing and designing the shape of the turbine wheel disc of the gas turbine based on bayesian optimization and GCN provided by the invention comprises the following steps:
1. performing parameterization modeling on the turbine wheel disc geometry of the gas turbine to be optimized by adopting a Bezier curve, determining the geometric parameter serving as a design variable, and simultaneously determining the value range of the geometric parameter to construct an optimization space of the geometric design variable of the wheel disc;
The expression of the Bezier curve is:
Wherein P (t) is a point on a Bezier curve, P i is a Bezier curve control point, B i,n (t) is a Bernstein basis function, t is a curve point generation parameter, n is a Bezier curve order, and t continuously changes in a range of [0,1] to determine a corresponding Bezier curve after the curve order n and the control point P i are determined.
The invention adopts the Bezier curve to describe the wheel disc molded line instead of the traditional method of describing the wheel disc molded line by using discrete points, and the discrete point method can describe the molded line simply, but does not have the advantages of less control parameters of the Bezier curve, flexible adjustment and infinite continuity, and a large number of data points are needed to describe the complex shape of the wheel disc finely.
The method for sectionally parameterizing the wheel disc adopts Bezier curves with different orders to parameterize different parts of the wheel disc, and combines good shape control capability of the Bezier curves, so that the wheel disc has enough optimization possibility on the premise of ensuring that geometrical parameters of key parts such as end face teeth are not changed.
Considering that the turbine wheel of the gas turbine has a combination relation between stages, the head and tail control points of the Bezier curve are fixed, so that the applicability of the geometric shape of the wheel established in the optimization process is ensured, and the number of design variables is reasonably reduced.
Referring to FIG. 2, considering that the turbine wheel disk part of the gas turbine is in an axisymmetric structure, two-dimensional optimization design is only carried out for the wheel surface molded line, the head and tail control points of the Bezier curve are fixed in consideration of the combination relation between the stages of the turbine wheel disk of the gas turbine, the transverse and longitudinal seats of the ith control point are marked as [ x i,yi ] in the middle of the Bezier curve, the coordinates of all the control points are used as geometric design variables, and the value range is +/-20% of the initial value, so that a design variable optimization space is formed. And parameterizing different parts of the wheel disc by adopting Bezier curves with different orders respectively.
2. Sampling in an optimization space formed by coordinates of each intermediate control point of a Bezier curve by using a Latin hypercube sampling method to obtain a wheel disc sample set S, establishing the Bezier curve for each sample in the sample set S to obtain a wheel disc geometric model, and calling finite element calculation software to perform grid division and unidirectional thermosetting coupling numerical calculation to obtain a wheel disc displacement field f 1 and a stress field f 2, thereby obtaining the maximum radial deformation delta u x,max and the maximum stress value sigma max of the wheel disc, simultaneously obtaining the corresponding wheel disc mass m by combining the wheel disc material parameters, deriving the wheel disc grid node coordinates C= [ X, y ], and jointly forming a database [ X ] = { S, C, f 1,f2,Δux,max,σmax, m } of the optimization design of the wheel disc geometric structure by using design variables and target parameters.
According to the invention, the Latin hypercube sampling method is adopted to sample the design variable in the optimization space, and the obtained sample set can fully describe the optimization space, so that the constructed wheel disc database is beneficial to training a prediction model with good performance.
It should be noted that the radial deformation of the wheel disc mentioned in the present invention refers to the radial displacement difference of the rim portion.
The unidirectional thermosetting coupling is performed during the finite element calculation analysis, namely, the temperature gradient load from the high-temperature fuel gas under the working condition of the wheel disc is considered. Most of the prior researches on optimal design of the wheel disc do not consider the heat load, which is obviously unreasonable, so that the obtained optimal result meets the design requirement only when no heat load exists, the actual operability is not achieved, or the design margin is too large, the optimal design is not called optimal, and the true optimal structure can be obtained only by optimizing under the condition of fitting the actual running condition of the wheel disc.
3. Normalizing the data set according to the formula:
Where [ X j ] is the j-th dataset in the database, min and Max represent the maximum and minimum values of the respective dimension data in the corresponding dataset, respectively, and ε=1×10 -6 is a small amount.
Generating random numbers will normalize the data setRandomly sorting and dividing the training sets into training sets according to the proportion of 7:3And a verification set
4. Displacement field stress field prediction model GNet is constructed based on a graph roll-up neural network, and with reference to fig. 3, gnet is composed of an input layer, a graph roll-up layer and an output layer, wherein the input layer is composed of a full-connection layer and an activation function GELU, the graph roll-up layer is composed of a 6-layer graph convolution operator and an activation function GELU, and the output layer is a 1-layer graph convolution operator. GNet inputting the sample set S and the wheel disc profile grid node coordinate C, and outputting the predicted displacement field of the wheel disc profileStress fieldThe network mapping relation is as follows:
in the formula, The method comprises the steps of obtaining a predicted displacement field or stress field of a wheel disc molded surface, wherein F is a graph convolution map, S is single sample data comprising the abscissa of a Bezier curve control point, C is grid node coordinates of a single sample, and Θ is a parameter to be learned of a network.
Using the data set obtained in step 3AndTraining is carried out, smooth average absolute deviation SmoothL is adopted as a loss function in the training process, the optimization algorithm of the network model adopts a stable version Adamax of the self-adaptive moment estimation algorithm, the initial learning rate is 0.001, the learning rate in the training process adopts a step-down strategy, the learning rate in the [100,150,200] step is respectively reduced to one tenth of that before, and finally the high-precision displacement field stress field prediction model GNet is obtained through training.
GNet adopts a Gaussian deviation linear unit GELU as an activation function, random regularization is introduced into the activation function, probability description is carried out on neuron input, and overfitting can be effectively restrained, and generalization capability of a model is enhanced. The method has the advantages that the wheel disc stress field prediction model is built based on the graph convolution neural network, the direct prediction of the displacement value and the stress value of the grid node of the wheel disc finite element model can be realized, the method can be suitable for any grid structure, the defect that the field prediction is not performed by using the convolution neural network and is based on regular grid nodes is overcome, and the method has extremely strong friendliness to field analysis and calculation of complex molded line physical models such as the turbine wheel disc of the gas turbine.
The obtained predicted displacement field and stress field are not intermediate quantities in the optimization process, and have important significance, on one hand, the visualized field information has strong interpretability which is not available in other proxy models, and on the other hand, the method monitors and judges the stress concentration phenomenon possibly occurring at the key parts of the wheel disc in the optimization design process by means of the field information.
Based on the obtained predicted stress field, monitoring and judging the stress concentration phenomenon of the key part of the wheel disc structure, specifically comprising the following steps:
In the formula, sigma loc,max is a local stress maximum value of a key part of the structure, sigma loc,m is a local stress average value, sigma th is a stress threshold value for stress concentration monitoring, and K is a local structure stress concentration factor.
Firstly, determining a stress threshold sigma th based on turbine wheel disc materials of a gas turbine and working conditions of high temperature, high temperature gradient and high rotating speed, secondly, monitoring stress values of key parts of a wheel disc structure based on an obtained predicted stress field, judging the stress concentration degree if the maximum stress value sigma loc,max exceeds sigma th, and finally, calculating a local structure stress concentration factor K by sigma loc,max and sigma loc,m to represent the stress concentration degree of the key parts.
5. And carrying out loop iteration of the wheel disc molded line design by adopting a Bayesian optimization method with a prior function of Gaussian process regression and an acquisition function of STABLE-EI and combining the high-precision wheel disc displacement field stress field prediction model GNet obtained through the training. The Bayesian optimization automatically optimizes the wheel disc molded line data, the prediction model predicts the stress field of the displacement field according to the newly generated wheel disc molded line to obtain the distribution of the displacement field, the maximum radial deformation and the stress field distribution and the maximum stress value,
Simultaneously, the mass of the wheel disc can be obtained by combining material parameters, and under the target requirements that the maximum radial deformation is smaller than the allowable radial deformation, the maximum stress is smaller than the allowable stress and the mass is minimum, and no obvious stress concentration phenomenon exists, the optimal structural design scheme of the wheel disc is finally obtained by continuously recommending and evaluating.
The stress field prediction of the wheel disc displacement field is carried out by using a high-precision prediction model obtained based on graph convolution neural network training, so that a time-consuming process that finite element software needs to be repeatedly called for analysis in the optimizing process in the traditional method is replaced, and a quick, accurate and efficient scheme is provided for the optimization design of the wheel disc structure.
The STABLE-EI is an improved STABLE form based on any variance of an expected increment EI widely used for acquiring a function in standard Bayesian optimization, and the function can be optimized to a STABLE region in an unstable region around a false peak so as to converge to a STABLE peak value, so that the method has good robustness, and the Bayesian optimization process is easier to obtain a globally optimal solution.
Wherein ,vt=σt(x,ΣX);zt=[mt(x,∑X)-ωσt,a(x,∑x)-f(x+)]/vt;Φ(z) is a standard normal cumulative distribution function, phi (z) is a standard normal probability density function, m (x) is a mean function, omega is a weight for penalizing the midpoint of the unstable region, f (x) is a function from gaussian process regression, and x+= argmaxf (x i).
The STABLE-EI is an acquisition function based on arbitrary variance improvement, and an unstable region around a false spike can optimize the function to a STABLE region to converge to a STABLE peak with the same computational complexity as EI in standard Bayesian optimization without having a tendency to converge to an unstable spike as in standard Bayesian optimization.
When a Bayesian optimization method is used for generating new sample points in the optimizing process, a high-precision prediction model obtained based on graph convolution neural network training is used for predicting the stress field of the wheel disc displacement field, so that the time-consuming process that finite element software is required to be repeatedly called for calculation in the optimizing process in the traditional method is replaced, and a quick, accurate and efficient scheme is provided for the optimization design of the wheel disc structure.
Examples
The invention relates to a shape optimization design method of a turbine wheel disc of a gas turbine based on Bayesian optimization and GCN, which is used for optimally designing the turbine wheel disc of the gas turbine, and specifically comprises the following steps:
1. As shown in FIG. 2, in order to ensure that geometric parameters of key parts such as end face teeth are unchanged, an 8-order Bezier curve and a 5-order Bezier curve are respectively adopted to carry out sectional parametric modeling on the wheel disc.
The expression of the Bezier curve is:
Wherein P (t) is a point on a Bezier curve, P i is a Bezier curve control point, B i,n (t) is a Bernstein basis function, t is a curve point generation parameter, n is a Bezier curve order, and t continuously changes in a range of [0,1] to determine a corresponding Bezier curve after the curve order n and the control point P i are determined.
Considering that the turbine wheel of the gas turbine has a combination relation between the stages, the head and tail control points of the Bezier curve are fixed, so that the number of variable control points in the middle of the Bezier curve is 7 and 4 respectively, thereby realizing fine description of the part to be optimized and fully excavating the optimization depth. And marking the transverse and longitudinal coordinates of the ith control point in the middle of the Bezier curve as [ x i,yi ], taking the coordinates of all the control points as geometric design variables, wherein the value ranges of the transverse coordinates and the longitudinal coordinates are respectively +/-10% and +/-20% of the initial values, and the value ranges of the design variables are shown in a table 1 to form an optimization space of the geometric design variables of the wheel disc.
Table 1 roulette design variable values
2. Sampling in an optimization space formed by coordinates of each intermediate control point of a Bezier curve by using a Latin hypercube sampling method to obtain a wheel disc sample set S, establishing the Bezier curve for each sample in the sample set S to obtain a wheel disc geometric model, and calling finite element calculation software to perform grid division and unidirectional thermosetting coupling numerical calculation to obtain a wheel disc displacement field f 1 and a stress field f 2, thereby obtaining the maximum radial deformation delta u x,max and the maximum stress value sigma max of the wheel disc, simultaneously obtaining the corresponding wheel disc mass m by combining the wheel disc material parameters, deriving the wheel disc grid node coordinates C= [ X, y ], and jointly forming a database [ X ] { S, C=, f 1,f2,Δux,max,σmax, m } of the optimization design of the wheel disc geometric structure by using design variables and target parameters.
3. Normalizing the data set according to the formula:
Where [ X j ] is the j-th dataset in the database, min and Max represent the maximum and minimum values of the respective dimension data in the corresponding dataset, respectively, and ε=1×10 -6 is a small amount.
Generating random numbers will normalize the data setRandomly sorting and dividing the training sets into training sets according to the proportion of 7:3And a verification set
4. Using the data set obtained in step3AndThe training of the displacement field stress field prediction model GNet (refer to fig. 3) is completed, the loss curve of the training process refers to fig. 4, and the cloud image of the predicted displacement field stress field and the real displacement field stress field obtained by the prediction model refers to fig. 5, wherein the real field, the predicted field and the error field are respectively from left to right, and the radial displacement field and the stress field are respectively from top to bottom, so that the prediction effect can be seen to be excellent.
Based on the obtained predicted stress field, monitoring and judging the stress concentration phenomenon of the key part of the wheel disc structure, specifically comprising the following steps:
In the formula, sigma loc,max is a local stress maximum value of a key part of the structure, sigma loc,m is a local stress average value, sigma th is a stress threshold value for stress concentration monitoring, and K is a local structure stress concentration factor.
Firstly, determining a stress threshold sigma th based on turbine wheel disc materials of a gas turbine and working conditions of high temperature, high temperature gradient and high rotating speed, secondly, monitoring stress values of key parts of a wheel disc structure based on an obtained predicted stress field, judging the stress concentration degree if the maximum stress value sigma loc,max exceeds sigma th, and finally, calculating a local structure stress concentration factor K by sigma loc,max and sigma loc,m to represent the stress concentration degree of the key parts.
5. And carrying out loop iteration of the wheel disc molded line design by adopting a Bayesian optimization method with a prior function of Gaussian process regression and an acquisition function of STABLE-EI and combining the high-precision wheel disc displacement field stress field prediction model GNet obtained through the training.
The Bayesian optimization automatically optimizes the wheel disc molded line data, the prediction model GNet predicts the stress field of the displacement field according to the newly generated wheel disc molded line, the displacement field distribution, the maximum radial deformation amount, the stress field distribution and the maximum stress value are obtained, the predicted displacement field stress field grid node scatter diagram of the new geometric structure in the optimization design process is respectively the radial displacement field and the stress field from top to bottom according to the figure 6, and meanwhile, the quality of the wheel disc can be obtained by combining material parameters.
And referring to various values of the optimization constraint parameters and the target parameters of the wheel disc shown in the table 2, continuously recommending and evaluating the values under the target requirements that the maximum radial deformation is smaller than the allowable radial deformation, the maximum stress is smaller than the allowable stress and the mass is minimum and no obvious stress concentration phenomenon exists, so that the optimal structural design scheme of the wheel disc is finally obtained.
TABLE 2 optimization constraint parameters and target parameters for roulette
While the invention has been described in detail in the foregoing general description and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that modifications and improvements can be made thereto. Accordingly, such modifications or improvements may be made without departing from the spirit of the invention and are intended to be within the scope of the invention as claimed.