Troubleshooting FSI Simulations: Solving Load Mapping & Time Step Synchronization Issues

CFD-CSD Coupling Transient Analysis

Executing accurate Fluid-Structure Interaction (FSI) simulations requires precise data transfer between computational fluid dynamics (CFD) solvers and finite element analysis (FEA) structural solvers. However, setting up transient unidirectional or bidirectional coupling often introduces temporal misalignment. Below is a breakdown of two common time-stepping anomalies and how to fix them.

Issue 1: Odd-Numbered Time Steps Skipped During Pressure Load Import

Symptom: Out of 120 export steps recorded in the fluid solver, only 60 load steps successfully map to the structural model—all odd-numbered steps (1, 3, 5, etc.) are skipped during data import.

Visualizing Load Mapping Behavior:

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Green = Imported Red = Skipped Step

Root Cause: This typically occurs due to mismatched export sampling frequencies, file-naming convention mismatches, or structural solver auto-skip rules:

• Write Frequency Mismatch: The CFD solver is configured to output data every 2 steps instead of every single time step.
• Zero-Padding/Index Parsers: File readers (e.g., in ANSYS System Coupling or Abaqus Co-Simulation) misinterpret file index suffixes if odd steps lack uniform digit padding (e.g., press_1.dat vs press_001.dat).
• Sub-step Downsampling: The structural load manager is set to import loads at matching physical time intervals, but defaults to skipping steps if physical timestamps overlap microsecond tolerance thresholds.

Recommended Solution

Check the CFD export control panel to ensure output frequency is set to 1. Verify that imported pressure filenames follow strict index padding rules. In the FEA side's Imported Load settings, set Data Sourcing to "All Available Steps" and disable automatic data downsampling.

Issue 2: Excessive Initial Time Span & Step Mismatch

Symptom: Auto time stepping is disabled, and the time step increment matches the unsteady CFD setup. However, configuring the end time of each structural load step based on CFD timestamp outputs causes an excessively large initial time step (from step 0 to step 1).

Timeline Gap Analysis:

Large Unstable Time Span (Initial ΔT)

Δt = CFD Step

t = 0.0s (Static Init) t = 0.5s (First CFD Export) t = 0.51s

Root Cause: This occurs due to an initial time offset between steady-state initialization and transient execution:

• Physical Time Offset: CFD calculations often start from an initialization time (or run transiently for several iterations to stabilize flow before data export begins at a non-zero time).
• Structural Reference Point: The FEA solver assumes step 1 starts at t = 0.0. If your first CFD data point is imported at t = 0.5s, the structural solver attempts to bridge 0.0s to 0.5s in a single load step, creating severe convergence failures.

Recommended Solution

Shift the time origin of your CFD exported dataset so that your first export matches step increment time. If the CFD run required pre-convergence initialization, zero out the physical clock using a post-processing script before importing files into the structural solver. Alternatively, insert an initial dummy structural step at t = start_time to absorb the initial gap.

Fluid-Structure Interaction CFD FEA Coupling Pressure Mapping Transient Analysis Time Step Convergence ANSYS Workbench / Abaqus