Domain: Engineering — Hydraulic Engineering
Target concept: Density Stratification

Video generation prompt: A transparent glass container is filled with clear water. A vertical glass tube with a small hole at the bottom is inserted into the water, and colored saltwater (with a density greater than the clear water) is injected into the tube. Operation: open the small hole at the bottom of the glass tube and record what happens to the colored saltwater after it is released into the clear water.

Evaluation guide:
(i) Target concept: Gravitational (density) instability between two fully miscible liquids of different density (colored saltwater and clear fresh water), where diffusive mixing across the interface is slow relative to the advective timescale of the denser fluid's descent, so the saltwater sinks as a coherent plume rather than mixing away uniformly.
Minimum mechanism: Colored saltwater must be objectively denser than clear water; tube must be open at bottom to allow unimpeded egress; water must be initially quiescent; container must be deep enough to allow observable descent; no external forces (e.g., stirring, vibration) may interfere.
(ii)Storyline: [Frame 0–1] Tube fully inserted, colored liquid statically confined inside tube above water level; clear water fills container around tube. [Frame 2–3] Hole opens → immediate localized egress begins. [Frame 4–7] Colored liquid forms descending plume aligned beneath tube axis, retaining integrity and sharp leading edge while advancing downward. [Frame 8–10] Plume reaches container base and accumulates as a distinct dense layer; upper clear water remains largely undisturbed with visible stratified interface. Perspective: Fixed frontal view capturing full tube length, water depth, and plume trajectory; no occlusion; lighting enables clear contrast between colored plume and clear water.

Manual:
Experimental Procedure
1. Place transparent glass container filled with clear water on stable surface.
2. Insert vertical glass tube (with small hole at bottom end) into water until tube stands upright and partially submerged.
3. Fill tube with colored saltwater (denser than clear water).
4. Open the small hole at tube bottom to release colored saltwater.
Observation of Experimental Phenomena
Colored saltwater exits tube and sinks vertically as a cohesive plume, maintaining distinct color and sharp interface with surrounding clear water; it does not rise, disperse upward, or mix uniformly upon release.
Summary of Experimental Principles
Density stratification: the higher-density colored saltwater sinks under gravity through the lower-density clear water and settles into a distinct layer at the bottom. Although the two liquids are fully miscible, molecular diffusion across the interface is slower than the time needed for the denser fluid to descend and settle, so a visibly sharp interface persists during the descent before the liquids eventually mix over a longer timescale.

Prompt Grounding rubric:
level_1: No transparent glass container visible; no vertical glass tube present; no colored liquid shown; no clear water background; no hole at tube bottom visible; no injection or opening action depicted.
level_2: At least one of: transparent container OR vertical tube OR colored liquid OR clear water appears, but all core objects are incomplete or misidentified (e.g., opaque container, plastic tube, uncolored liquid, or no water); no discernible hole; no injection or opening action shown.
level_3: Transparent glass container AND vertical glass tube inserted into clear water AND colored liquid inside tube AND small hole visible at tube bottom AND action of opening the hole is visually indicated (e.g., cap removal, valve turn, or sudden onset of flow); all objects present and correctly typed.
level_4: All elements from score 3 present AND tube remains vertically fixed throughout; colored liquid is confined within tube before opening; clear water occupies majority of container volume outside tube; hole location is unambiguously at bottom end of tube (not side or top).
level_5: All elements from score 4 present AND tube is fully immersed with top above water surface; colored liquid is visibly contained inside tube prior to opening; hole is centered at lowest point of tube's submerged end; no extraneous objects (e.g., hands, labels, tools) occlude core setup; viewpoint is frontal and stable, showing full tube length and container depth.

Scientific and Causal Correctness rubric:
level_1: Colored saltwater rises or disperses upward instead of sinking; no density-driven downward motion occurs; the color spreads evenly through the clear water almost immediately with no distinguishable plume or boundary; the hue fades away rather than staying visible.
level_2: Colored saltwater moves downward somewhat after release but does not hold together as a plume: it spreads sideways near the tube's hole, stalls partway down, or drifts back upward instead of continuing to sink; little or no visible contrast remains between the colored liquid and the surrounding clear water.
level_3: The colored saltwater exits the hole and descends as a recognizable column or plume beneath the tube, keeping a distinct color and a visible boundary with the clear water, and continues sinking past the point where it exits the tube without reversing direction or fully mixing away before nearing the container floor.
level_4: All 3-point criteria are met, AND the plume stays roughly beneath the tube's hole as it falls, without drifting sideways or splitting into separate streams before reaching the bottom; the plume's leading edge stays visually distinguishable from the surrounding clear water for at least half of its descent, only beginning to blur noticeably as it nears the container floor; no large-scale mixing occurs before the plume approaches the bottom.
level_5: All 4-point criteria are met, AND the plume descends in one continuous motion without stopping, splitting apart, or breaking into separate droplets; the color intensity of the descending saltwater looks consistent from release to settling; the boundary between the colored saltwater and clear water stays visually distinct from release until the plume settles at the bottom, with no colored liquid swirling back upward into the clear water above.

Spatiotemporal Consistency rubric:
level_1: The glass tube or container appears, disappears, or changes shape between frames; the water surface tears or disconnects; the tube bends or shifts position discontinuously; the highlight on the glass tube or the shadow cast by the container jumps to a different position or intensity between one frame and the next, inconsistent with a single fixed light source; motion of the colored saltwater stutters with visible skips or frozen frames; the liquid appears as a solid, blocky mass rather than a fluid plume.
level_2: The tube or container shows slight jitter or wobble in position; the descending plume blurs intermittently in a way inconsistent with its falling speed; one isolated moment where the highlight on the glass doesn't match the direction of the apparent light source; a brief freeze (under a second) or visible interpolation artifact in the plume's motion; the glass texture warps slightly without the tube or container breaking apart.
level_3: The container and tube stay rigid, upright, and continuously present throughout, with no sudden vanishing, reappearing, or breaking apart; the water surface outside the plume stays flat and calm; the descending colored saltwater moves smoothly without strobing, snapping, or sudden warping; the glass surfaces keep a consistent level of transparency and refraction throughout the clip.
level_4: All 3-point conditions are also met, AND camera framing stays fixed with no panning, tilting, or zooming; there is no mismatched parallax between the tube and the container walls as time passes; reflections on the glass stay geometrically consistent from frame to frame; the plume's downward speed changes gradually (e.g., a slight acceleration as it falls) rather than jumping abruptly between speeds.
level_5: All 4-point conditions are also met, AND refraction at the water surface and glass walls looks visually consistent throughout (e.g., the submerged part of the tube appears offset at the waterline in the same way from frame to frame); the plume's outer boundary stays visually crisp rather than blurring into a diffuse haze throughout the descent, with no doubled or ghost-like edges appearing from one frame to the next; the tube and container's wall thickness and curvature look uniform for the entire sequence.