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Adding excess stress terms into the momentum equations |
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August 20, 2024, 10:24 |
Adding excess stress terms into the momentum equations
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#1 |
New Member
Murat Can ONEN
Join Date: Sep 2021
Location: Turkey
Posts: 13
Rep Power: 5 |
Hello
I am trying to add x and y momentum source terms that include second gradients of the velocity field. My UDF is as follows but does not compile when I wrote equations as such: Code:
DEFINE_SOURCE(momentum_x_source, c, t, dS, eqn) { real source; real grad_Txx_e[ND_ND]; real grad_Txy_e[ND_ND]; real grad_DUDX[ND_ND]; real grad_DUDY[ND_ND]; real grad_DVDX[ND_ND]; /* Calculate the gradients of Txx and DUDX */ grad_Txx_e = C_UDSI_G(c, t, Txx); grad_Txy_e = C_UDSI_G(c, t, Txy); grad_DUDX = C_DUDX_G(c, t); grad_DUDY = C_DUDY_G(c, t); grad_DVDX = C_DVDX_G(c, t); /* Source term for the x-momentum equation */ source = grad_Txx_e[0] - 2 * M_Eta_S * grad_DUDX[0] + grad_Txy_e[1]- M_Eta_S * (grad_DUDY [1] + grad_DVDX [0]); dS[eqn] = 0.0; // Set the derivative of the source term if needed return source; } Thanks, mcanoenen |
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August 22, 2024, 05:25 |
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#2 |
New Member
Murat Can ONEN
Join Date: Sep 2021
Location: Turkey
Posts: 13
Rep Power: 5 |
Hello, I want to share an update about my progress
APROACH 1: Deleting second-order velocity gradients from the equations (the basic way) Based on my understanding, taking the second derivative of the velocity field was not feasible without defining new User-Defined Scalars. Therefore, I reintegrated the viscous terms into the Txx, Txy, and Tyy. This allowed me to incorporate the derivatives of Txx_, Txy_, and Tyy_ into the momentum equations, albeit with a risk to convergence. To ensure convergence, I modified the viscosity using an exponential equation that gradually adjusted the momentum equations, starting from a viscosity of 1 Pa.s and approaching a minimum of 1e-6, as given by exp(-0.01*ITER_N), while maintaining a constant viscosity value within the Txx_, Txy_, and Tyy_ terms. Ultimately, the solution demonstrated satisfactory convergence for both the momentum (velocity residuals) and pressure correction (mass residual) equations. Code:
DEFINE_SOURCE(momentum_x_source, c, t, dS, eqn) { real source; /* Source term for the x-momentum equation */ source = C_UDSI_G(c, t, Txx)[0] + C_UDSI_G(c, t, Txy)[1]; // x-component of the gradient dS[eqn] = 0.0; // Set the derivative of the source term if needed return source; } DEFINE_SOURCE(momentum_y_source, c, t, dS, eqn) { real source; /* Source term for the y-momentum equation */ source = C_UDSI_G(c, t, Tyy)[1] + C_UDSI_G(c, t, Txy)[0]; // y-component of the gradient dS[eqn] = 0.0; // Set the derivative of the source term if needed return source; } Code:
DEFINE_DIFFUSIVITY(artificial_diff, c, t, i) { return fmax(1e-4, pow(2.71, -0.001 * N_ITER)); } DEFINE_PROPERTY(viscosity, c, t) { return fmax(1e-6, * pow(2.71, -0.001 * N_ITER)); } Last edited by mcanoenen; September 4, 2024 at 07:31. |
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September 4, 2024, 07:30 |
Second update
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#3 |
New Member
Murat Can ONEN
Join Date: Sep 2021
Location: Turkey
Posts: 13
Rep Power: 5 |
APPROACH 2) Inclusion of second-order velocity gradient terms with defining new UDSs. (Answer to the query title)
While Approach 1 has given results, I can confirm that the velocity profile lacked the viscous characteristics at the corners of the velocity profile. To avoid this, the second-order terms can be solved with the momentum equation and removed with the source terms as in the original form shared here. So I have defined the following User Defined Scalars to do this. Code:
enum //defining new UDSs// { DUUDXX, DUUDYY, DVVDXX, DVVDYY } DEFINE_EXECUTE_ON_LOADING(define_variables, libname) { Set_User_Scalar_Name(DUUDXX, "DUUDXX"); Set_User_Scalar_Name(DUUDYY, "DUUDYY"); Set_User_Scalar_Name(DVVDXX, "DVVDXX"); Set_User_Scalar_Name(DVVDYY, "DVVDYY"); } Code:
DEFINE_SOURCE(DUUDYY_source, c, t, dS, eqn) { real source; real x[ND_ND]; source = C_UDSI(c, t, DUUDYY) - C_MU_L(c, t) * C_DUDY(c, t); dS[eqn] = 1; // Set the derivative of the source term return source; } DEFINE_SOURCE(DVVDXX_source, c, t, dS, eqn) { real source; real x[ND_ND]; source = C_UDSI(c, t, DVVDXX) - C_MU_L(c, t) * C_DVDX(c, t); dS[eqn] = 1; // Set the derivative of the source term return source; } DEFINE_SOURCE(DVVDYY_source, c, t, dS, eqn) { real source; real x[ND_ND]; source = C_UDSI(c, t, DVVDYY) - C_MU_L(c, t) * C_DVDY(c, t); dS[eqn] = 1; // Set the derivative of the source term return source; } Code:
DEFINE_SOURCE(momentum_x_source, c, t, dS, eqn) { real source; /* Source term for the x-momentum equation */ source = - C_UDSI_G(c, t, DUUDXX)[0] - C_UDSI_G(c, t, DUUDYY)[1]; // remove second order derivative from x momentum dS[eqn] = 0.0; // Set the derivative of the source term if needed return source; } DEFINE_SOURCE(momentum_y_source, c, t, dS, eqn) { real source; /* Source term for the y-momentum equation */ source = - C_UDSI_G(c, t, DVVDXX)[0] - C_UDSI_G(c, t, DVVDYY)[1] ; // remove second order derivative from Y momentum dS[eqn] = 0.0; // Set the derivative of the source term if needed return source; } Regards, mcanoenen. |
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fluent, udf, viscoelasticfluid |
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