momentumsolvers.c

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#include "momentumsolvers.h"
 
 
#undef __FUNCT__
#define __FUNCT__ "ComputeTotalResidual"
/**
 * @brief Internal helper implementation: `ComputeTotalResidual()`.
 * @details Local to this translation unit.
 */
static PetscErrorCode ComputeTotalResidual(UserCtx *user)
{
    PetscErrorCode ierr;
    SimCtx *simCtx = user->simCtx;
    
    // Extract Time Parameters from Context
    const PetscInt  ti      = simCtx->step;
    const PetscInt  tistart = simCtx->StartStep;
    const PetscReal dt      = simCtx->dt;
 
    PetscFunctionBeginUser;
    PROFILE_FUNCTION_BEGIN;
    // 1. Calculate Spatial Terms (stored in user->Rhs)
    //    Rhs = -Div(Flux) + Viscous + Source
    ierr = ComputeRHS(user, user->Rhs); CHKERRQ(ierr);
 
    // 2. Add Physical Time Derivative Terms (BDF Discretization)
    //    The equation solved is: dU/dtau = RHS_Spatial + RHS_Temporal
    
    if (COEF_TIME_ACCURACY > 1.1 && ti != tistart && ti != 1) {
        // --- BDF2 (Second Order Backward Difference) ---
        // Formula: (1.5*U^{n} - 2.0*U^{n-1} + 0.5*U^{n-2}) / dt
        // Moved to RHS: -1.5/dt * U^{n} + ...
        ierr = VecAXPY(user->Rhs, -COEF_TIME_ACCURACY/dt, user->Ucont);
        ierr = VecAXPY(user->Rhs, +2.0/dt, user->Ucont_o);
        ierr = VecAXPY(user->Rhs, -0.5/dt, user->Ucont_rm1);
    } else {
        // --- BDF1 (First Order / Euler Implicit) ---
        // Formula: (U^{n} - U^{n-1}) / dt
        // Moved to RHS: -1.0/dt * U^{n} + ...
        ierr = VecAXPY(user->Rhs, -1.0/dt, user->Ucont);
        ierr = VecAXPY(user->Rhs, +1.0/dt, user->Ucont_o);
    }
 
    // 3. Enforce Boundary Conditions on the Residual
    ierr = EnforceRHSBoundaryConditions(user); CHKERRQ(ierr);
 
    PROFILE_FUNCTION_END;
    PetscFunctionReturn(0);
}
 
 
#undef __FUNCT__
#define __FUNCT__ "Momentum_Solver_Explicit_RungeKutta4"
/**
 * @brief Internal helper implementation: `MomentumSolver_Explicit_RungeKutta4()`.
 * @details Local to this translation unit.
 */
PetscErrorCode MomentumSolver_Explicit_RungeKutta4(UserCtx *user, IBMNodes *ibm, FSInfo *fsi)
{
    PetscErrorCode ierr;
    (void)fsi;
    // --- Context Acquisition ---
    // Get the master simulation context from the first block's UserCtx.
    // This is the bridge to access all former global variables.
    SimCtx *simCtx = user[0].simCtx;
    PetscReal dt = simCtx->dt;
    //PetscReal st = simCtx->st;
    PetscInt  istage;
    PetscReal alfa[] = {0.25, 1.0/3.0, 0.5, 1.0};
 
    PetscFunctionBeginUser;
 
    PROFILE_FUNCTION_BEGIN;
 
    LOG_ALLOW(GLOBAL, LOG_INFO, "Executing explicit momentum solver (Runge-Kutta) for %d block(s).\n",simCtx->block_number);
 
    // --- 1. Pre-Loop Initialization (Legacy Logic) ---
    // This block prepares boundary conditions and allocates the RHS vector for all blocks
    // before the main RK loop begins. This logic is preserved from the original code.
    LOG_ALLOW(GLOBAL, LOG_DEBUG, "Preparing all blocks for Runge-Kutta solve...\n");
    for (PetscInt bi = 0; bi < simCtx->block_number; bi++) {
 
        ierr = ApplyBoundaryConditions(&user[bi]); CHKERRQ(ierr);
 
        /*
        // Immersed boundary interpolation (if enabled)
        if (simCtx->immersed) {
            LOG_ALLOW(LOCAL, LOG_DEBUG, "  Performing pre-RK IBM interpolation for block %d.\n", bi);
            for (PetscInt ibi = 0; ibi < simCtx->NumberOfBodies; ibi++) {
                // The 'ibm' and 'fsi' pointers are passed directly from FlowSolver
                ierr = ibm_interpolation_advanced(&user[bi], &ibm[ibi], ibi, 1); CHKERRQ(ierr);
            }
        }
        */
        
        // Allocate the persistent RHS vector for this block's context
        ierr = VecDuplicate(user[bi].Ucont, &user[bi].Rhs); CHKERRQ(ierr);
    }
    LOG_ALLOW(GLOBAL, LOG_DEBUG, "Pre-loop initialization complete.\n");
 
    // --- 2. Main Runge-Kutta Loop ---
    // The legacy code had an outer `pseudot` loop that only ran once. We preserve it.
    for (PetscInt pseudot = 0; pseudot < 1; pseudot++) {
        // Loop over each block to perform the RK stages
        for (PetscInt bi = 0; bi < simCtx->block_number; bi++) {
            for (istage = 0; istage < 4; istage++) {
                LOG_ALLOW(LOCAL, LOG_DEBUG, "  Block %d, RK Stage %d (alpha=%.4f)...\n", bi, istage, alfa[istage]);
 
                // a. Calculate the Right-Hand Side (RHS) of the momentum equation.
                ierr = ComputeRHS(&user[bi], user[bi].Rhs); CHKERRQ(ierr);
 
                // b. Advance Ucont to the next intermediate stage using the RK coefficient.
                //    Ucont_new = Ucont_old + alpha * dt * RHS
                ierr = VecWAXPY(user[bi].Ucont, alfa[istage] * dt, user[bi].Rhs, user[bi].Ucont_o); CHKERRQ(ierr);
 
                // c. Update local ghost values for the new intermediate Ucont.
                ierr = DMGlobalToLocalBegin(user[bi].fda, user[bi].Ucont, INSERT_VALUES, user[bi].lUcont); CHKERRQ(ierr);
                ierr = DMGlobalToLocalEnd(user[bi].fda, user[bi].Ucont, INSERT_VALUES, user[bi].lUcont); CHKERRQ(ierr);
 
                // d. Re-apply boundary conditions for the new intermediate velocity.
                //    This is crucial for the stability and accuracy of the multi-stage scheme.
                ierr = ApplyBoundaryConditions(&user[bi]); CHKERRQ(ierr);
                
            } // End of RK stages for one block
 
            /*
            // Final IBM Interpolation for the block (if enabled)
            if (simCtx->immersed) {
                LOG_ALLOW(LOCAL, LOG_DEBUG, "  Performing post-RK IBM interpolation for block %d.\n", bi);
                for (PetscInt ibi = 0; ibi < simCtx->NumberOfBodies; ibi++) {
                    ierr = ibm_interpolation_advanced(&user[bi], &ibm[ibi], ibi, 1); CHKERRQ(ierr);
                }
            }
            */
 
        } // End loop over blocks
 
        // --- 3. Inter-Block Communication (Legacy Logic) ---
        // This is called after all blocks have completed their RK stages.
        if (simCtx->block_number > 1) {
      //    LOG_ALLOW(GLOBAL, LOG_DEBUG, "Updating multi-block interfaces after RK stages.\n");
        //   ierr = Block_Interface_U(user); CHKERRQ(ierr);
        }
 
    } // End of pseudo-time loop
 
    // --- 4. Cleanup ---
    // Destroy the RHS vectors that were created at the start of this function.
    for (PetscInt bi = 0; bi < simCtx->block_number; bi++) {
        ierr = VecDestroy(&user[bi].Rhs); CHKERRQ(ierr);
    }
    
    LOG_ALLOW(GLOBAL, LOG_INFO, "Runge-Kutta solve completed for all blocks.\n");
 
    PROFILE_FUNCTION_END;
 
    PetscFunctionReturn(0);
}
 
#undef __FUNCT__
#define __FUNCT__ "MomentumSolver_DualTime_Picard_RK4"
/**
 * @brief Internal helper implementation: `MomentumSolver_DualTime_Picard_RK4()`.
 * @details Local to this translation unit.
 */
PetscErrorCode MomentumSolver_DualTime_Picard_RK4(UserCtx *user, IBMNodes *ibm, FSInfo *fsi)
{
    (void)ibm;
    (void)fsi;
    // --- CONTEXT ACQUISITION BLOCK ---
    SimCtx *simCtx = user[0].simCtx;
    const PetscInt block_number = simCtx->block_number;
    
    // Legacy Names (Physics/Time) - Kept for brevity in formulas
    const PetscInt ti = simCtx->step;
    const PetscReal dt = simCtx->dt;       // Physical Time Step
    const PetscReal cfl = simCtx->pseudo_cfl;     // Initial CFL Scaling
    const PetscReal alfa[] = {0.25, 1.0/3.0, 0.5, 1.0}; // RK4 Coefficients
    Vec *pRhs; // Per-block backup of Rhs at last accepted pseudo-state.
 
    // State flags
    PetscBool force_restart = PETSC_FALSE;
 
    // Renamed Solver Parameters
    const PetscInt  max_pseudo_steps = simCtx->mom_max_pseudo_steps; // Max Dual-Time Iterations
    const PetscReal tol_abs_delta    = simCtx->mom_atol; // Stop if |dU| < tol
    const PetscReal tol_rtol_delta   = simCtx->mom_rtol; // Stop if |dU|/|dU0| < tol
    // --- END CONTEXT ACQUISITION BLOCK ---
 
    PetscErrorCode ierr;
    PetscMPIInt    rank;
    PetscInt       istage, pseudo_iter;
    PetscReal      ts, te, cput;
 
    // --- Global Convergence Metrics ---
    PetscReal global_norm_delta = 10.0; 
    PetscReal global_rel_delta  = 1.0;  
    PetscReal global_rel_resid  = 1.0;  
 
    // --- Local Metric Arrays (Per Block) ---
    PetscReal *delta_sol_norm_init, *delta_sol_norm_prev, *delta_sol_norm_curr, *delta_sol_rel_curr;
    PetscReal *resid_norm_init,     *resid_norm_prev,     *resid_norm_curr,     *resid_rel_curr;
    PetscReal *pseudo_dt_scaling;
 
    PetscFunctionBeginUser;
    PROFILE_FUNCTION_BEGIN;
    ierr = MPI_Comm_rank(PETSC_COMM_WORLD, &rank); CHKERRQ(ierr);
    LOG_ALLOW(GLOBAL, LOG_INFO, "Executing Dual-Time Momentum Solver for %d block(s).\n", block_number);
 
    // --- Allocate metric arrays ---
    ierr = PetscMalloc2(block_number, &delta_sol_norm_init, block_number, &delta_sol_norm_prev); CHKERRQ(ierr);
    ierr = PetscMalloc2(block_number, &delta_sol_norm_curr, block_number, &delta_sol_rel_curr); CHKERRQ(ierr);
    ierr = PetscMalloc2(block_number, &resid_norm_init,     block_number, &resid_norm_prev); CHKERRQ(ierr);
    ierr = PetscMalloc2(block_number, &resid_norm_curr,     block_number, &pseudo_dt_scaling); CHKERRQ(ierr);
    ierr = PetscMalloc1(block_number, &resid_rel_curr); CHKERRQ(ierr);
    ierr = PetscMalloc1(block_number, &pRhs); CHKERRQ(ierr);
 
    ierr = PetscTime(&ts); CHKERRQ(ierr);
 
    // --- 1. Pre-Loop Initialization ---
    
    if (block_number > 1) {
      // ierr = Block_Interface_U(user); CHKERRQ(ierr);
    }
 
    for (PetscInt bi = 0; bi < block_number; bi++) {
        ierr = ApplyBoundaryConditions(&user[bi]); CHKERRQ(ierr);
        
        // Immersed boundary interpolation (if enabled)
 
        // Allocate workspace
        ierr = VecDuplicate(user[bi].Ucont, &user[bi].Rhs); CHKERRQ(ierr);
        ierr = VecDuplicate(user[bi].Rhs, &pRhs[bi]); CHKERRQ(ierr);
        ierr = VecDuplicate(user[bi].Ucont, &user[bi].dUcont); CHKERRQ(ierr);
        ierr = VecDuplicate(user[bi].Ucont, &user[bi].pUcont); CHKERRQ(ierr);
        
        // Initialize Backup (pUcont) with current state
        ierr = VecCopy(user[bi].Ucont, user[bi].pUcont); CHKERRQ(ierr);
 
        // --- Calculate Initial Total Residual (Spatial + Temporal) ---
        ierr = ComputeTotalResidual(&user[bi]); CHKERRQ(ierr);
 
        // Backup Rhs with current state
        ierr = VecCopy(user[bi].Rhs, pRhs[bi]); CHKERRQ(ierr);
 
        // Compute Initial Norms
        ierr = VecNorm(user[bi].Rhs, NORM_INFINITY, &resid_norm_init[bi]); CHKERRQ(ierr);
        
        // Initialize history for backtracking logic
        resid_norm_prev[bi]     = resid_norm_init[bi]; // Meaningful ratio on first iteration
        delta_sol_norm_prev[bi] = 1000.0;
        pseudo_dt_scaling[bi] = cfl; // Initialize adaptive scalar
 
        LOG_ALLOW(GLOBAL,LOG_INFO," Block %d | Max RHS = %.6f | Pseudo-CFL = %.4f .\n", bi, resid_norm_init[bi], cfl);
    }
    
    // --- 2. Main Pseudo-Time Iteration Loop ---
    pseudo_iter = 0;
    while ((force_restart || (global_norm_delta > tol_abs_delta || global_rel_delta > tol_rtol_delta) || pseudo_iter < 1)
            && pseudo_iter < max_pseudo_steps)
    {
        pseudo_iter++;
        force_restart = PETSC_FALSE;
    
        for (PetscInt bi = 0; bi < block_number; bi++) {
            
            // === 4-Stage Explicit Runge-Kutta Loop ===
            for (istage = 0; istage < 4; istage++) {
                
                LOG_ALLOW(GLOBAL,LOG_TRACE," Pseudo-Iter: %d | RK-Stage: %d\n", pseudo_iter, istage);
                
                // RK Update: U_new = U_old + (Scaler * Alpha * dt) * Total_Residual
                ierr = VecWAXPY(user[bi].Ucont, 
                                pseudo_dt_scaling[bi] * alfa[istage] * dt, 
                                user[bi].Rhs, 
                                user[bi].pUcont); CHKERRQ(ierr);
 
                // Sync Ghosts & Re-apply BCs for intermediate stage
                ierr = UpdateLocalGhosts(&user[bi],"Ucont"); CHKERRQ(ierr);
                
                //ierr = DMGlobalToLocalBegin(user[bi].fda, user[bi].Ucont, INSERT_VALUES, user[bi].lUcont); CHKERRQ(ierr);
                //ierr = DMGlobalToLocalEnd(user[bi].fda, user[bi].Ucont, INSERT_VALUES, user[bi].lUcont); CHKERRQ(ierr);
                
                ierr = ApplyBoundaryConditions(&user[bi]); CHKERRQ(ierr);
 
                // --- Re-calculate Total Residual for next stage ---
                ierr = ComputeTotalResidual(&user[bi]); CHKERRQ(ierr);
      
            } // End RK Stages
 
            // Immersed boundary interpolation (if enabled)
 
            // === Convergence Metrics Calculation ===
            
            // Calculate dU = U_current - U_backup
            ierr = VecWAXPY(user[bi].dUcont, -1.0, user[bi].pUcont, user[bi].Ucont); CHKERRQ(ierr);
 
            // Compute Infinity Norms
            ierr = VecNorm(user[bi].dUcont, NORM_INFINITY, &delta_sol_norm_curr[bi]); CHKERRQ(ierr);
            ierr = VecNorm(user[bi].Rhs, NORM_INFINITY, &resid_norm_curr[bi]); CHKERRQ(ierr);
        
            // Normalize relative metrics
            if (pseudo_iter == 1) {
                delta_sol_norm_init[bi] = delta_sol_norm_curr[bi];
                delta_sol_rel_curr[bi]  = 1.0;
                resid_rel_curr[bi]      = 1.0;
                // resid_norm_init[bi] set correctly before the loop — do not overwrite
            } else {
                if (delta_sol_norm_init[bi] > 1.0e-10) 
                    delta_sol_rel_curr[bi] = delta_sol_norm_curr[bi] / delta_sol_norm_init[bi];
                else 
                    delta_sol_rel_curr[bi] = 0.0;
 
                if(resid_norm_init[bi] > 1.0e-10) 
                    resid_rel_curr[bi] = resid_norm_curr[bi] / resid_norm_init[bi];
                else 
                    resid_rel_curr[bi] = 0.0;
            }
          
            // --- File Logging ---
            if (!rank) {
                FILE *f;
                char filen[PETSC_MAX_PATH_LEN + 128];
                ierr = PetscSNPrintf(filen, sizeof(filen), "%s/Momentum_Solver_Convergence_History_Block_%1d.log", simCtx->log_dir, bi); CHKERRQ(ierr);
                if(simCtx->step == simCtx->StartStep + 1 && pseudo_iter == 1 && !simCtx->continueMode) f = fopen(filen, "w");
                else f = fopen(filen, "a");
                
                PetscFPrintf(PETSC_COMM_WORLD, f, "Step: %d | PseudoIter(k): %d| | Pseudo-cfl: %.4f |dUk|: %le | |dUk|/|dUprev|: %le | |Rk|: %le | |Rk|/|Rprev|: %le \n",
                             (int)ti, (int)pseudo_iter, pseudo_dt_scaling[bi], delta_sol_norm_curr[bi], delta_sol_rel_curr[bi], resid_norm_curr[bi],resid_rel_curr[bi]);
                fclose(f);
            }
        } // End loop over blocks
 
        // --- Update Global Convergence Criteria ---
        global_norm_delta = -1.0e20; 
        global_rel_delta  = -1.0e20; 
        global_rel_resid  = -1.0e20;
        
        for (PetscInt bi = 0; bi < block_number; bi++) {
            global_norm_delta = PetscMax(delta_sol_norm_curr[bi], global_norm_delta);
            global_rel_delta  = PetscMax(delta_sol_rel_curr[bi],  global_rel_delta);
            global_rel_resid  = PetscMax(resid_rel_curr[bi],      global_rel_resid);
        }
        ierr = PetscTime(&te); CHKERRQ(ierr);
        cput = te - ts;
        LOG_ALLOW(GLOBAL, LOG_INFO, "  Pseudo-Iter(k) %d: |dUk|=%e, |dUk|/|dU0| = %e, |Rk|/|R0| = %e, CPU=%.2fs\n",
              pseudo_iter, global_norm_delta, global_rel_delta, global_rel_resid, cput);
 
        // === Backtracking Line Search ===
        for (PetscInt bi = 0; bi < block_number; bi++) {
 
            PetscReal resid_ratio = resid_norm_curr[bi] / resid_norm_prev[bi];
            
            PetscBool is_sol_nan, is_resid_nan, is_diverging;
            // Check for NaNs in Solution and Residual
            is_sol_nan = (PetscBool)PetscIsNanReal(delta_sol_norm_curr[bi]);
            is_resid_nan = (PetscBool)PetscIsNanReal(resid_norm_curr[bi]);
 
            // TRIGGER: Divergence Detected
            is_diverging = (PetscBool)(pseudo_iter > 1 && 
                resid_norm_curr[bi] > simCtx->mom_dt_rk4_residual_norm_noise_allowance_factor*resid_norm_prev[bi] && 
                delta_sol_norm_curr[bi] > delta_sol_norm_prev[bi]
                ); 
            
                if(is_diverging || is_sol_nan || is_resid_nan) {
 
                    if(is_sol_nan){
                        LOG_ALLOW(LOCAL, LOG_WARNING, "  Block %d: NaN detected in solution update norm.\n", bi);
                    }
                    if(is_resid_nan){
                        LOG_ALLOW(LOCAL, LOG_WARNING, "  Block %d: NaN detected in residual norm.\n", bi);
                    }
                    if(is_diverging){
                        LOG_ALLOW(LOCAL, LOG_WARNING, "  Block %d: Divergence detected (|R| and |dU| increasing).\n", bi);
                    }
 
                    if(pseudo_dt_scaling[bi] < simCtx->min_pseudo_cfl) {
                        SETERRQ(PETSC_COMM_WORLD, PETSC_ERR_CONV_FAILED, 
                            "Block %d: Solver diverged (NaN or explosion) and min time-step reached. Simulation aborted.", bi);
                    }
 
                    // For nan situations, we always backtrack.
                    force_restart = PETSC_TRUE;
 
                // 1. Reduce Step Size
                pseudo_dt_scaling[bi] *= 0.5*simCtx->pseudo_cfl_reduction_factor; // Aggressive deceleration (1/2 x reduction factor).
                // NOTE: pseudo_iter-- is handled once after this loop to avoid multi-decrement for block_number > 1.
 
                LOG_ALLOW(LOCAL, LOG_DEBUG, "  Block %d: Backtracking triggered. Reducing scaling to %f.\n", bi, pseudo_dt_scaling[bi]);
                
                // 3. RESTORE from Backup
                ierr = VecCopy(user[bi].pUcont, user[bi].Ucont); CHKERRQ(ierr);
 
                ierr = VecCopy(pRhs[bi], user[bi].Rhs); CHKERRQ(ierr);
                
                // 4. SYNC Ghost Cells
                ierr = UpdateLocalGhosts(&user[bi],"Ucont"); CHKERRQ(ierr);
                //ierr = DMGlobalToLocalBegin(user[bi].fda, user[bi].Ucont, INSERT_VALUES, user[bi].lUcont); CHKERRQ(ierr);
                //ierr = DMGlobalToLocalEnd(user[bi].fda, user[bi].Ucont, INSERT_VALUES, user[bi].lUcont); CHKERRQ(ierr);
 
                // 5. Reset current norms to avoid polluting next loop check.
                delta_sol_norm_curr[bi] = 1000.0;
                resid_norm_curr[bi] = 1000.0;
 
            } 
            else {
                // SUCCESS: Step accepted. Update Backup and History.
                ierr = VecCopy(user[bi].Ucont, user[bi].pUcont); CHKERRQ(ierr);
                ierr = VecCopy(user[bi].Rhs,   pRhs[bi]);        CHKERRQ(ierr);
                resid_norm_prev[bi]     = resid_norm_curr[bi];
                delta_sol_norm_prev[bi] = delta_sol_norm_curr[bi];
 
                // Adaptive Ramping of solution speed (Pseudo-CFL)
 
                PetscBool is_converging_fast;  
                is_converging_fast = (PetscBool)(resid_ratio < 0.9);
                PetscBool is_stagnating;       
                is_stagnating = (PetscBool)(resid_ratio >= 0.98 && resid_ratio < 1.0);
                PetscBool is_noisy;            
                is_noisy = (PetscBool)(resid_ratio >= 1.0 && resid_ratio < simCtx->mom_dt_rk4_residual_norm_noise_allowance_factor);
                
                PetscReal current_cfl = pseudo_dt_scaling[bi];
                PetscReal cfl_floor = 10*simCtx->min_pseudo_cfl; // Define a "low CFL" threshold for more aggressive acceleration.
                PetscReal cfl_ceiling = 0.75*simCtx->max_pseudo_cfl; // Define a "high CFL" threshold for mild acceleration.
 
                if(is_converging_fast){ 
                    // Residual is falling fast (good convergence)
                    pseudo_dt_scaling[bi] *= simCtx->pseudo_cfl_growth_factor;
                } else if(is_stagnating && current_cfl < cfl_floor){ 
                    // Residual is falling very slow (weak convergence.) we accelerate at low CFL
                    pseudo_dt_scaling[bi] *= PetscMin(1.2*simCtx->pseudo_cfl_growth_factor, 1.1); // more aggressive acceleration at low CFL
                } else if(is_stagnating && current_cfl >= cfl_floor && current_cfl < cfl_ceiling && !is_noisy){ 
                    // moderate CFL - normal acceleration
                    pseudo_dt_scaling[bi] *= simCtx->pseudo_cfl_growth_factor;
                } else if(is_stagnating && current_cfl >= cfl_ceiling && !is_noisy){ 
                    // high CFL - mild acceleration (creep)
                    pseudo_dt_scaling[bi] *= PetscMax(0.95*simCtx->pseudo_cfl_growth_factor,1.005);
                } else if(is_noisy){ 
                    // Residual is rising slightly (within noise allowance). we decelerate mildly.
                    if(current_cfl > cfl_ceiling){
                        // High CFL - more aggressive deceleration
                        pseudo_dt_scaling[bi] *= PetscMax(0.9*simCtx->pseudo_cfl_reduction_factor,0.9);
                    } else if(current_cfl > cfl_floor && current_cfl <= cfl_ceiling){
                        // Moderate CFL - mild deceleration
                        pseudo_dt_scaling[bi] *= simCtx->pseudo_cfl_reduction_factor;
                    } else if(current_cfl <= cfl_floor){
                        // Low CFL - gentle acceleration
                        // rise in residual likely due to noise, so we back off slightly.
                        pseudo_dt_scaling[bi] *= PetscMax(0.95*simCtx->pseudo_cfl_growth_factor,1.01);
                    }
                } 
                // Clamp to max bound.
                pseudo_dt_scaling[bi] = PetscMin(pseudo_dt_scaling[bi], simCtx->max_pseudo_cfl);
                pseudo_dt_scaling[bi] = PetscMax(pseudo_dt_scaling[bi], simCtx->min_pseudo_cfl);
            }
        }
 
        // Decrement once regardless of how many blocks backtracked.
        if (force_restart) pseudo_iter--;
 
        if (block_number > 1) {
             // ierr = Block_Interface_U(user); CHKERRQ(ierr);
        }
    } // End while loop
 
 
    // Smart CFL Carry Over
 
    PetscReal local_min_cfl = 1.0e20;
    PetscReal global_min_cfl;
 
    PetscReal cfl_floor = 10*simCtx->min_pseudo_cfl;
 
    // 1. Find the "Weakest Link" (lowest CFL) across local blocks
    for (PetscInt bi = 0; bi < block_number; bi++) {
        if (pseudo_dt_scaling[bi] < local_min_cfl) {
            local_min_cfl = pseudo_dt_scaling[bi];
        }
    }
 
    // 2. MPI Reduction (Crucial if blocks are distributed across ranks)
    // We want the minimum CFL across the entire simulation, not just this processor.
    ierr = MPI_Allreduce(&local_min_cfl, &global_min_cfl, 1, MPIU_REAL, MPI_MIN, PETSC_COMM_WORLD); CHKERRQ(ierr);
 
    // 3. Apply Safety Factor & Update simCtx
    // We use a factor (e.g., 0.85) to back off slightly for the start of the next timestep
    // to accommodate the initial residual spike from the BDF term.
    
    PetscReal cfl_carry_over_relaxation_factor = 1.0; // Could be parameterized in simCtx if desired.
    
    PetscReal next_start_cfl;
 
    if(global_min_cfl < cfl_floor){
        // If the minimum CFL is very low, we back off more aggressively.
        next_start_cfl = cfl_floor * 1.5;
    } else{
        next_start_cfl = global_min_cfl * cfl_carry_over_relaxation_factor;
    }
    
    next_start_cfl = PetscMin(next_start_cfl, simCtx->max_pseudo_cfl);
    next_start_cfl = PetscMax(next_start_cfl, simCtx->min_pseudo_cfl);
 
    // 5. Save for next Physical Time Step
    if (!rank) {
        LOG_ALLOW(GLOBAL, LOG_INFO, "  CFL Adaptation: Step %d finished at CFL %.3f. Next step will start at CFL %.3f.\n", 
                  simCtx->step, global_min_cfl, next_start_cfl);
    }
    
    // Update the context so the next call to MomentumSolver reads this new value
    simCtx->pseudo_cfl = next_start_cfl;    
 
    // --- Final Cleanup ---
    for (PetscInt bi = 0; bi < block_number; bi++) {
        ierr = VecDestroy(&user[bi].Rhs); CHKERRQ(ierr);
        ierr = VecDestroy(&user[bi].dUcont); CHKERRQ(ierr);
        ierr = VecDestroy(&user[bi].pUcont); CHKERRQ(ierr);
        ierr = VecDestroy(&pRhs[bi]); CHKERRQ(ierr);
    }
    ierr = PetscFree(pRhs); CHKERRQ(ierr);
 
    ierr = PetscFree2(delta_sol_norm_init, delta_sol_norm_prev);CHKERRQ(ierr);
    ierr = PetscFree2(delta_sol_norm_curr, delta_sol_rel_curr);CHKERRQ(ierr);
    ierr = PetscFree2(resid_norm_init,     resid_norm_prev);CHKERRQ(ierr);
    ierr = PetscFree2(resid_norm_curr,     pseudo_dt_scaling); CHKERRQ(ierr);
    ierr = PetscFree(resid_rel_curr); CHKERRQ(ierr);
    
    LOG_ALLOW(GLOBAL, LOG_INFO, "Momentum Solver converged/completed after %d pseudo-iterations.\n", pseudo_iter);
    PROFILE_FUNCTION_END;
    PetscFunctionReturn(0);
}