On the unsteady characteristics of turbulent separations over a forward–backward-facing step
Bibliographic record
Abstract
Turbulent separation bubbles over and behind a two-dimensional forward–backward-facing step submerged in a deep turbulent boundary layer are investigated using a time-resolved particle image velocimetry. The Reynolds number based on the step height and free-stream velocity is 12 300, and the ratio of the streamwise length to the height of the step is 2.36. The upstream turbulent boundary layer thickness is 4.8 times the step height to ensure a strong interaction of the upstream turbulence structures with the separated shear layers over and behind the step. The velocity measurements were performed in streamwise–vertical planes at the channel mid-span and streamwise–spanwise planes at various vertical distances from the wall. The unsteady characteristics of the separation bubbles and their associated turbulence structures are studied using a variety of techniques including linear stochastic estimation, proper orthogonal decomposition and variable-interval time averaging. The results indicate that the low-frequency flapping motion of the separation bubble over the step is induced by the oncoming large-scale alternating low- and high-velocity streaky structures. Dual separation bubbles appear periodically over the step at a higher frequency than the flapping motion, and are attributed to the inherent instability in the rear part of the mean separation bubble. The separation bubble behind the step exhibits a flapping motion at the same frequency as the separation bubble over the step, but with a distinct phase delay. At instances when an enlarged separation bubble is formed in front of the step, a pair of vertical counter-rotating vortices is formed in the immediate vicinity of the leading edge.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".