TwoFluidPipe Reporting and Validation

This page describes how to extract engineering results from neqsim.process.equipment.pipeline.TwoFluidPipe for long multiphase flowlines and how to compare those results with external simulator or field data.

Current reporting status

TwoFluidPipe exposes detailed profile and summary APIs. The convenience class neqsim.process.equipment.pipeline.twophasepipe.reporting.TwoFluidPipeReport builds standard CSV, text, JSON, event, and benchmark comparison outputs from those APIs.

The recommended production workflow is:

Build and run the TwoFluidPipe model.
Extract spatial profiles from the pipe, or call TwoFluidPipeReport helper methods.
Add summary metrics and flow-assurance indicators.
Export to CSV/JSON for plotting, review, or comparison.
If available, compare against traceable experimental, field, or external-simulator data using the benchmark harness. Keep model-to-model checks distinct from experimental validation.

Steady-state profile reporting

After pipe.run(), the following methods provide one value per pipe section:

API	Unit	Description
`getPositionProfile()`	m	Section midpoint positions
`getPressureProfile()`	Pa	Pressure profile
`getTemperatureProfile()`	K	Fluid temperature profile
`getTemperatureProfile("C")`	degC	Fluid temperature profile in Celsius
`getLiquidHoldupProfile()`	fraction	Total liquid holdup
`getWaterCutProfile()`	fraction	Water fraction of liquid
`getOilHoldupProfile()`	fraction	Oil holdup
`getWaterHoldupProfile()`	fraction	Water holdup
`getGasVelocityProfile()`	m/s	Gas velocity
`getLiquidVelocityProfile()`	m/s	Liquid velocity
`getOilVelocityProfile()`	m/s	Oil velocity when oil-water slip is active
`getWaterVelocityProfile()`	m/s	Water velocity when oil-water slip is active
`getOilWaterSlipProfile()`	m/s	Oil velocity minus water velocity
`getFlowRegimeProfile()`	enum	Gas-liquid flow regime by section
`getOilWaterFlowRegimeProfile()`	enum	Oil-water flow configuration by section
`getWaterWettingProfile()`	boolean	Water-wetting indicator for corrosion screening
`getWaterDropoutRiskProfile()`	boolean	Water dropout / accumulation risk
`getEntrainmentFractionProfile()`	fraction	Estimated liquid entrainment in annular/mist flow
`getEntrainedDropletDiameterProfile()`	m	Characteristic entrained droplet diameter
`getInclinedSectionGasCarryoverNumberProfile()`	dimensionless	Local uphill liquid-carryover screen; not a system stability criterion
`getInclinedSectionLiquidFallbackPotentialProfile()`	boolean	Local fallback flag from the carryover screen
`getSevereSluggingNumberProfile()`	dimensionless	Deprecated alias for the local carryover screen
`getSevereSlugPotentialProfile()`	boolean	Result flag from the most recent explicit flowline-riser evaluation
`getHeatTransferProfile()`	W/(m2 K)	Heat-transfer coefficient profile, when configured
`getSurfaceTemperatureProfile()`	K	Ambient/surface temperature profile, when configured

Example Java extraction:

pipe.run();

double[] x = pipe.getPositionProfile();
double[] pressurePa = pipe.getPressureProfile();
double[] temperatureC = pipe.getTemperatureProfile("C");
double[] liquidHoldup = pipe.getLiquidHoldupProfile();
double[] waterCut = pipe.getWaterCutProfile();
double[] gasVelocity = pipe.getGasVelocityProfile();
double[] liquidVelocity = pipe.getLiquidVelocityProfile();
PipeSection.FlowRegime[] regimes = pipe.getFlowRegimeProfile();
double[] entrainment = pipe.getEntrainmentFractionProfile();
double[] gasCarryoverNumber = pipe.getInclinedSectionGasCarryoverNumberProfile();
boolean[] localFallback = pipe.getInclinedSectionLiquidFallbackPotentialProfile();
boolean[] waterWetting = pipe.getWaterWettingProfile();

for (int i = 0; i < x.length; i++) {
  double pBara = pressurePa[i] * 1.0e-5;
  System.out.printf("%8.1f,%10.4f,%8.3f,%8.5f,%8.5f,%8.3f,%8.3f,%s%n",
      x[i], pBara, temperatureC[i], liquidHoldup[i], waterCut[i],
      gasVelocity[i], liquidVelocity[i], regimes[i]);
}

The same steady-state results can be exported directly:

String profileCsv = TwoFluidPipeReport.toSteadyStateProfileCsv(pipe);
String summaryText = TwoFluidPipeReport.toSummaryText(pipe);
String summaryJson = TwoFluidPipeReport.toSummaryJson(pipe);
String eventsCsv = TwoFluidPipeReport.toSlugAndFlowAssuranceCsv(pipe);

Transient reporting

For transient cases, call runTransient(dt, id) repeatedly and store a snapshot at the reporting interval required by the study. Do not store every internal sub-step for long pipelines unless high-frequency pressure waves or slug arrivals are being investigated.

UUID id = UUID.randomUUID();
double dt = 1.0;
int reportEvery = 10;

for (int step = 0; step < 3600; step++) {
  pipe.runTransient(dt, id);

  if (step % reportEvery == 0) {
    double time = pipe.getSimulationTime();
    double[] x = pipe.getPositionProfile();
    double[] p = pipe.getPressureProfile();
    double[] hL = pipe.getLiquidHoldupProfile();
    // Write one row per position with this time stamp.
  }
}

Recommended transient CSV columns:

time_s,position_m,pressure_bara,temperature_C,liquid_holdup,water_cut,
gas_velocity_m_s,liquid_velocity_m_s,oil_velocity_m_s,water_velocity_m_s,
flow_regime,oil_water_flow_regime,water_wetting,water_dropout_risk,
entrainment_fraction,entrained_droplet_diameter_m,
inclined_section_gas_carryover_number,inclined_section_liquid_fallback_potential,
severe_slugging_number,severe_slug_potential

Thermal-energy validation

For a closed-boundary thermal runTransient(...), call getLastThermalEnergyBalanceReport() to validate the same post-step sensible-energy model that changed the fluid and wall temperatures. Its discrete balance is

\[\Delta E_f+\Delta E_w=E_{adv}+E_{JT}-E_{amb}\]

where positive $E_{adv}$ and $E_{JT}$ add energy and positive $E_{amb}$ removes energy. CLOSED external faces contribute zero sensible advection, but internal face transport remains active. In the multilayer model, the fluid and first wall layer use the same instantaneous heat rate; the reported ambient loss is the last-layer flux from the same explicit update.

pipe.setHeatTransferCoefficient(25.0); // W/(m2 K); the report is null when heat transfer is disabled
pipe.runTransient(0.001, UUID.randomUUID());
TwoFluidThermalEnergyBalanceReport thermal = pipe.getLastThermalEnergyBalanceReport();
boolean closes = thermal.isWithinTolerance(1.0e-5, 1.0e-10);

The report is null when heat transfer is disabled. It covers fluid sensible energy and simple-wall or radial-layer storage. Strict domain-level closure applies only when both external mass boundaries are CLOSED and phase transfer does not change material inventory. Open boundaries require explicit boundary-enthalpy terms; phase-changing cases additionally require compositional and latent-energy terms. In those cases this report is an internal post-step temperature-model diagnostic, not a full domain energy audit. For a closed cooldown, also verify zero boundary mass/enthalpy transport, monotonic all-cell cooling without ambient undershoot, repeatability, serialization/copy behavior, both explicit and IMEX paths, and mesh/time-step refinement.

Phase-transfer validation

When setIncludeMassTransfer(true) is enabled, validate gas, oil, and water separately rather than checking total mass alone. Condensation is assigned from the equilibrium oil/aqueous mass split, whereas evaporation is withdrawn from the actual donor inventories. The transfer-only requirements are:

\[\Gamma_G+\Gamma_O+\Gamma_W=0\]

and, using donor velocity for transferred momentum,

\[S_{p,G}+S_{p,O}+S_{p,W}=0\]

Use TwoFluidMassBalanceReport to check GAS, OIL, WATER, LIQUID, and TOTAL. A useful phase-transition test starts from a cell with no liquid seed, crosses the SRK/CPA dew point in both directions, and sweeps at least three nearby temperatures on each side. Report the EOS, mixing rule, composition, absolute pressure, temperature, relaxation time, time step, mesh, and phase inventories. Repeat the run to verify deterministic behavior and compare a refined time step and mesh. Serialize a condensed-state copy and require the original and copy to follow the same reheating trajectory. As a negative control, repeat the cooldown with setIncludeMassTransfer(false) and require every phase source and inventory change to remain zero even though the temperature crosses the dew point.

For an aqueous-first transition, the first condensation source must be water even though the gas-only hydrodynamic water cut defaults to zero. For an oil-first transition, the water source must remain zero. In a gas + oil + aqueous flash, the reported equilibrium liquid mass fractions must both be included and sum to one. FlashTable and rigorous-flash runs should give the same phase identity; use sufficiently fine tables near phase boundaries.

The phase-resolved closure conserves bulk phase inventories, but it does not yet transport a full component-composition vector in every hydrodynamic cell. Do not interpret it as commercial-simulator equivalence or use total-mass closure alone as validation of liquid identity.

The severe_slugging_number header is retained as a deprecated duplicate for CSV compatibility. It contains the local inclined-section gas-carryover number, not the explicit system stability result. New consumers should use inclined_section_gas_carryover_number. Call evaluateSevereSluggingSystem(...) before exporting if severe_slug_potential should contain a system-level classification.

Summary metrics

Use these methods for an executive summary or design report:

API	Description
`getInletPressure()`	Inlet pressure in bara
`getOutletPressure()`	Outlet pressure in bara
`getAverageLiquidHoldup()`	Volume-weighted average liquid holdup
`getDominantFlowRegime()`	Most frequent flow regime
`getAverageSuperficialGasVelocity()`	Average superficial gas velocity
`getAverageSuperficialLiquidVelocity()`	Average superficial liquid velocity
`getAverageMixtureDensity()`	Volume-weighted mixture density
`getMaxMixtureVelocity()`	Maximum mixture velocity
`getErosionalVelocity()`	API 14E erosional velocity
`getErosionalVelocityMargin(double)`	Maximum velocity divided by erosional velocity
`getFlowAnalysisSummary()`	Mid-pipe dimensionless flow summary
`getThermalSummary()`	Thermal model summary
`getSlugStatisticsSummary()`	Slug-tracking summary
`getHydrateRiskSections()`	Sections below configured hydrate temperature
`getWaxRiskSections()`	Sections below configured wax appearance temperature

Closure diagnostics

The two-fluid closure pass updates additional diagnostics that are useful for model review and validation. Each value is available both on TwoFluidSection and as a top-level TwoFluidPipe profile:

Section API	Pipe profile API	Description
`getOilWaterFlowRegime()`	`getOilWaterFlowRegimeProfile()`	Oil-water flow configuration
`isWaterWetting()`	`getWaterWettingProfile()`	Water-wetting indicator for corrosion screening
`isWaterDropoutRisk()`	`getWaterDropoutRiskProfile()`	Water dropout / accumulation risk
`getEntrainmentFraction()`	`getEntrainmentFractionProfile()`	Estimated liquid entrainment fraction
`getEntrainedDropletDiameter()`	`getEntrainedDropletDiameterProfile()`	Entrained droplet diameter
`getInclinedSectionGasCarryoverNumber()`	`getInclinedSectionGasCarryoverNumberProfile()`	Local uphill liquid-carryover screen
`isInclinedSectionLiquidFallbackPotential()`	`getInclinedSectionLiquidFallbackPotentialProfile()`	Local fallback flag
`getSevereSluggingNumber()`	`getSevereSluggingNumberProfile()`	Deprecated aliases for the same local screen
`isSevereSlugPotential()`	`getSevereSlugPotentialProfile()`	Last explicit flowline-riser system result

The steady-state and transient profile CSV exporters include these diagnostics. Boolean values are written as true or false; a missing oil-water regime is written as an empty field. The system-result profile is meaningful only after evaluateSevereSluggingSystem(...) and is cleared when the next transient step changes the solved state.

Benchmark comparison format

The validation harness reads external simulator or field data in this CSV format:

case,time_s,position_m,variable,value,abs_tolerance,rel_tolerance,source

Supported captured variables include:

pressure_pa
pressure_bara
temperature_k
liquid_holdup
water_cut
oil_holdup
water_holdup
gas_velocity_m_s
liquid_velocity_m_s
oil_velocity_m_s
water_velocity_m_s
entrainment_fraction
entrained_droplet_diameter_m
inclined_section_gas_carryover_number
inclined_section_liquid_fallback_flag
severe_slugging_number
water_wetting_flag
water_dropout_risk_flag
severe_slug_potential_flag

severe_slugging_number is the deprecated benchmark key for inclined_section_gas_carryover_number; it is retained only for existing comparison files.

Example use:

Path reference = Path.of("olga_export.csv");
List<TwoFluidBenchmarkHarness.BenchmarkPoint> points =
    TwoFluidBenchmarkHarness.readCsv(reference);

TwoFluidBenchmarkHarness.Snapshot snapshot = TwoFluidBenchmarkHarness.capture(pipe);
TwoFluidBenchmarkHarness.Comparison comparison =
    TwoFluidBenchmarkHarness.compare(snapshot, points);

if (!comparison.isPassed()) {
  throw new AssertionError(comparison.failureSummary());
}

For transient comparisons, capture and pass a list of snapshots. The harness uses linear interpolation in time and position for continuous profiles. Variables ending in _flag use nearest-neighbour sampling so boolean diagnostics remain 0.0 or 1.0. Intervals containing a non-finite diagnostic sentinel also use the nearest endpoint instead of producing NaN. Comparison results can be exported as CSV:

String comparisonCsv = TwoFluidPipeReport.toComparisonCsv(comparison);

Reporting recommendations for long flowlines

For long oil and gas flowlines, report at least:

Geometry and discretization: length, diameter, roughness, elevation profile, number of sections, mesh refinement strategy.
Boundary conditions: inlet stream, flow rate, outlet pressure, transient changes.
Thermodynamics: fluid model, flash interval, mass-transfer relaxation time, heat-transfer setup.
Pressure and temperature profiles.
Liquid holdup, water cut, phase velocity, flow-regime, entrainment, wetting, and severe-slugging profiles.
Slug statistics and terrain low-point liquid accumulation.
Hydrate/wax/thermal risk sections if thresholds are configured.
Erosional velocity margin and maximum mixture velocity.
Benchmark comparison table with source, assumptions, uncertainty, mesh/timestep study, and failed as well as passed metrics when external data are available.

Public severe-slugging evidence

SevereSluggingBenchmarkHarness reads the digitized public Tengesdal (2002) -3-degree velocity map and reports a confusion matrix without forcing transition observations into a binary class. Across 26 severe and 15 stable observations, the current Taitel system screen has 22 true positives, 4 false negatives, 8 false positives, and 7 true negatives (70.7% accuracy). The 14 transition observations are reported separately as 6 predicted severe and 8 predicted stable.

SevereSluggingExperimentalBenchmarkTest reproduces large-facility Test 3 with the physical flowline/riser geometry and RK4 integration. Severe slugging here is a deterministically chaotic limit cycle: a relative inlet-pressure perturbation of 1e-12, far below any experimental significance, changes the peak-to-peak riser-base pressure by more than a factor of two and the apparent period by more than a factor of 1.5. The benchmark therefore evaluates a four-member ensemble (12 and 16 sections, 0.1 and 0.2 s outer steps, and a perturbed trajectory) and separates reproducible from trajectory-sensitive evidence.

Reproducible across the ensemble: phase-resolved and total mass closure below 1e-15, a time-averaged riser-base pressure of 171-176 kPa that agrees within 4% across mesh, outer step and perturbation, and an outlet liquid rate that blows out above and falls back below the liquid feed rate in every realization.

Trajectory-sensitive and reported as a range only: peak-to-peak pressure 42-300 kPa versus 98 +/- 5 kPa digitized, apparent cycle period 14-35 s versus 38 +/- 2 s measured, and a maximum tracked outlet slug of 1.5-4.9 m (0.10-0.33 riser heights), below the experimental severe-slug definition. The modelled pressure swing brackets the measured amplitude, but the period is systematically short. These residuals must accompany any reported severe-slugging result; they are not hidden by a pass/fail summary.

The benchmark disables the steady-state wall-clock guard and asserts it did not fire, so results do not depend on how fast the executing machine is.

The source is S. Tengesdal’s 2002 BSEE report: https://www.bsee.gov/sites/bsee.gov/files/tap-technical-assessment-program/397aa.pdf.

Gaps and planned improvements

The current API is adequate for engineering studies and benchmark development. A polished industrial report workflow should still add:

Plot templates for pressure, temperature, holdup, water cut, flow regime, and slug events.
Direct import of OLGA/LedaFlow export formats where licensing allows it.