Convert symbolic PDE coefficients to
converts the symbolic objects of the structure
coeffs = pdeCoefficientsToDouble(
double-precision numbers or function handles. The output is a structure
coeffs that can then be used to define the coefficients of a PDE model
specifyCoefficients in Partial Differential Equation Toolbox™.
coeffs contains the coefficients
f for the PDE system with the form
that can be solved in Partial Differential Equation Toolbox. For more information, see Equations You Can Solve Using PDE Toolbox (Partial Differential Equation Toolbox).
Extract Coefficients From a Symbolic PDE
Create a symbolic PDE that represents the Laplacian of the variable
syms u(x,y) f pdeeq = laplacian(u,[x y]) == f
pdeeq(x, y) =
Extract the coefficients of the PDE as symbolic expressions and display their values.
symCoeffs = pdeCoefficients(pdeeq,u,'Symbolic',true); structfun(@disp,symCoeffs)
pdeCoefficients converts the symbolic PDE into a scalar PDE equation of the form
and extract the coefficients
f into the structure
Choose a value for
symCoeffs are symbolic objects, use
pdeCoefficientsToDouble to convert the coefficients to
double data type. The coefficients with
double data type are valid inputs for the
specifyCoefficients function in the PDE Toolbox.
symCoeffs = subs(symCoeffs,f,-3); coeffs = pdeCoefficientsToDouble(symCoeffs)
coeffs = struct with fields: a: 0 c: [4x1 double] m: 0 d: 0 f: -3
System of Several PDEs
Solve a system of two second-order PDEs. You can solve the PDE system by extracting the PDE coefficients symbolically using
pdeCoefficients, converting the coefficients to double-precision numbers using
pdeCoefficientsToDouble, and specifying the coefficients in the PDE model using
The system of PDEs represents the deflection of a clamped structural plate under a uniform pressure load. The system of PDEs with the dependent variables and is given by
where is the bending stiffness of the plate given by
and is the modulus of elasticity, is Poisson's ratio, is the plate thickness, is the transverse deflection of the plate, and is the pressure load.
Create a PDE model for the system of two equations.
model = createpde(2);
Create a square geometry. Specify the side length of the square. Then include the geometry in the PDE model.
len = 10.0; gdm = [3 4 0 len len 0 0 0 len len]'; g = decsg(gdm,'S1',('S1')'); geometryFromEdges(model,g);
Specify the values of the physical parameters of the system. Let the external pressure be a symbolic variable
pres that can take any value.
E = 1.0e6; h_thick = 0.1; nu = 0.3; D = E*h_thick^3/(12*(1 - nu^2)); syms pres
Declare the PDE system as a system symbolic equations. Extract the coefficients of the PDE and return them in symbolic form.
syms u1(x,y) u2(x,y) pdeeq = [-laplacian(u1) + u2; -D*laplacian(u2) - pres]; symCoeffs = pdeCoefficients(pdeeq,[u1 u2],'Symbolic',true)
symCoeffs = struct with fields: m: 0 a: [2x2 sym] c: [4x4 sym] f: [2x1 sym] d: 0
Display the coefficients
Substitute a value for
pres using the
subs function. Since the outputs of
subs are symbolic objects, use the
pdeCoefficientsToDouble function to convert the coefficients to the
double data type, which makes them valid inputs for the PDE Toolbox.
symCoeffs = subs(symCoeffs,pres,2); coeffs = pdeCoefficientsToDouble(symCoeffs)
coeffs = struct with fields: a: [4x1 double] c: [16x1 double] m: 0 d: 0 f: [2x1 double]
Specify the PDE coefficients for the PDE model.
specifyCoefficients(model,'m',coeffs.m,'d',coeffs.d, ... 'c',coeffs.c,'a',coeffs.a,'f',coeffs.f);
Specify spring stiffness. Specify boundary conditions by defining distributed springs on all four edges.
k = 1e7; bOuter = applyBoundaryCondition(model,'neumann','Edge',(1:4), ... 'g',[0 0],'q',[0 0; k 0]);
Specify the mesh size of the geometry and generate a mesh for the PDE model.
hmax = len/20; generateMesh(model,'Hmax',hmax);
Solve the model.
res = solvepde(model);
Access the solution at the nodal locations.
u = res.NodalSolution;
Plot the transverse deflection of the plate.
numNodes = size(model.Mesh.Nodes,2); figure; pdeplot(model,'XYData',u(1:numNodes),'contour','on') title 'Transverse Deflection'
Find the transverse deflection at the plate center.
wMax = min(u(1:numNodes,1))
wMax = -0.2763
Compare the result with the deflection at the plate center computed analytically.
pres = 2; wMax = -.0138*pres*len^4/(E*h_thick^3)
wMax = -0.2760
Specify Dependent Variables of PDE System
Create a PDE system of four equations with four dependent variables, , , , and .
syms A(x,y,z,t) E(x,y,z,t) P(x,y,z,t) T(x,y,z,t) eqn1 = diff(A,t) == -A*E*(-exp(T)); eqn2 = diff(P,t) == -P*E*(-exp(T)); eqn3 = diff(E,t) == -A*E*(-exp(T)) - P*E*(-exp(T)); eqn4 = diff(T,t) == laplacian(T,[x y z]) + A*E*(-exp(T)) + P*E*(-exp(T)); pdeeq = [eqn1; eqn2; eqn3; eqn4]
pdeeq(x, y, z, t) =
Extract the coefficients of the PDE system as symbolic expressions. Specify the variable
u to represent the dependent variables.
u = [A E P T]; symCoeffs = pdeCoefficients(pdeeq,u,'Symbolic',true)
symCoeffs = struct with fields: m: [4x4 sym] a: [4x4 sym] c: [12x12 sym] f: [4x1 sym] d: [4x4 sym]
Display the coefficients
Convert the coefficients to the
double data type so that they are valid inputs for the
specifyCoefficients function in Partial Differential Equation Toolbox. Because the
a coefficient in
symCoeffs contains the dependent variables and is not constant, calling
pdeCoefficientsToDouble(symCoeffs) without the second input argument can return an error. Instead, specify the second argument as the dependent variables
u when using
coeffs = pdeCoefficientsToDouble(symCoeffs,u)
coeffs = struct with fields: a: @makeCoefficient/coefficientFunction c: [144x1 double] m: 0 d: [16x1 double] f: 0
symCoeffs — Coefficients of PDE in symbolic form
structure of symbolic expressions
Coefficients of PDE in symbolic form, specified as a structure of symbolic expressions.
u — Dependent variables of PDE
Dependent variables of PDE, specified as a symbolic function. The argument
u must contain stationary or time-dependent variables in two or
syms E(x,y,z,t) B(x,y,z,t); u = [E B];
coeffs — Coefficients of PDE operating on doubles
structure of doubles and function handles
Coefficients of PDE operating on doubles, returned as a structure of
double-precision numbers and function handles as required by the
specifyCoefficients function. The fields of the structure are
f. For details on interpreting the
coefficients in the format required by Partial Differential Equation Toolbox, see:
pdeCoefficientsToDouble returns a coefficient as a function
handle, the function handle takes two structures as input arguments,
state, and returns double-precision
output. The function handle is displayed as
@makeCoefficient/coefficientFunction in the Command Window. To
display the formula of the function handle in
coeffs.f in symbolic
In some cases, not all generated coefficients can be used by
specifyCoefficients. For example, the
coefficient must take a special matrix form when
m is nonzero. For
more details, see d Coefficient When m Is Nonzero (Partial Differential Equation Toolbox).
Version HistoryIntroduced in R2021a
R2023a: Specify dependent variables when converting PDE coefficients
You can specify the dependent variables
u of a PDE system as the
second input argument of
pdeCoefficientsToDouble when converting symbolic
coefficients of the PDE system to double-precision numbers or function handles. For example,
see Specify Dependent Variables of PDE System.