Domain: Natural Science — Marine Science
Target concept: In a transparent flume, a saltwater-freshwater mixture similar to that in an estuary is used with dye as an indicator.

Video generation prompt: In a transparent flume, clear freshwater is pumped in from one end, while blue-dyed saltwater is pumped in from the other end, allowing the two water streams to meet and mix.

Evaluation guide:
(i) Target concept: Density-driven two-layer stratified flow in a confined channel, where saline water intrudes as a bottom layer beneath fresher water due to gravitational instability.
Minimum mechanism: Blue-dyed fluid must be denser than clear fluid (achieved via salinity, not just dye), resulting in sustained vertical separation at interface, with mixing governed by interfacial shear and buoyancy, not diffusion alone.
(ii)Storyline: Frame 1–3: Left-end freshwater stream fully enters field of view, laminar and colorless; Frame 4–6: Right-end blue-dyed saltwater stream fully enters field of view, laminar and uniformly blue; Frame 7–10: Streams meet centrally: immediate formation of inclined or horizontal interface with blue layer below; progressive intermingling begins at interface with fine-scale structure; viewpoint remains fixed, front-facing, orthogonal to flume axis; all elements remain fully within frame with no occlusion or cropping.

Manual:
Experimental Procedure
1. Place a transparent rectangular flume horizontally on a stable surface.
2. Pump clear freshwater continuously from the left end.
3. Pump blue-dyed saltwater continuously from the right end.
4. Observe where the two streams meet and mix in the center.
Observation of Experimental Phenomena
- Two distinct streams enter from opposite ends: left stream is colorless and transparent; right stream is uniformly blue.
- At the meeting point, the denser blue saltwater sinks beneath the freshwater, forming a visible horizontal interface.
- Gradual mixing occurs across the interface, with blue dye diffusing upward while freshwater intrudes downward in fine interleaved layers.
Summary of Experimental Principles
- Saltwater is denser than freshwater due to dissolved ions; this density difference drives stratified flow in estuaries.
- In a controlled flume, this mimics the salt wedge intrusion typical of partially mixed estuaries, visualized using dye as a passive tracer.

Prompt Grounding rubric:
level_1: No transparent flume visible; no freshwater stream present; no blue-dyed saltwater stream present; no two streams entering from opposite ends; no meeting or mixing region shown.
level_2: A transparent container is present but not a flume (e.g., beaker, bowl, or undefined vessel); one of the two water streams is present (either clear freshwater OR blue-dyed saltwater), but not both; streams do not enter from opposite ends; no visible point of contact between streams.
level_3: A transparent flume is present; both clear freshwater and blue-dyed saltwater streams are present; both streams enter the flume, one from the left end, one from the right end; streams meet at a central region; mixing region is visible as an interface zone where colors interact.
level_4: All conditions for score 3 are satisfied; additionally: the freshwater stream is visually distinct as completely colorless and transparent (no tint, no dye trace); the blue-dyed saltwater stream is uniformly and consistently blue (no fading, streaking, or unintended hue shift); the two streams maintain separate identity up to the point of contact before intermixing.
level_5: All conditions for score 4 are satisfied; additionally: the flume has flat parallel sidewalls and rectangular cross-section (standard flume geometry); freshwater enters exclusively from the left end with laminar flow appearance; blue-dyed saltwater enters exclusively from the right end with laminar flow appearance; the interface region shows continuous, side-by-side coexistence followed by progressive intermingling; no reversal, splitting, or premature homogenization before the central mixing zone.

Scientific and Causal Correctness rubric:
level_1: No scientifically plausible density-driven interface behavior; streams appear identical in color/transparency; mixing occurs instantly upon entry with no visible stratification, intrusion, or fingering; blue dye disperses uniformly into freshwater immediately without density contrast effect.
level_2: One stream is colored but not blue, or blue appears only as a surface film or floating blob; freshwater is visibly turbid or dyed; no observable difference in behavior at interface (e.g., both streams merge symmetrically without preferential sinking/overriding); no evidence of saltwater being denser than freshwater.
level_3: Blue-dyed saltwater stream is denser and sinks slightly below the freshwater stream upon contact; a visible horizontal or inclined interface forms where the two fluids meet; partial separation is maintained in the initial contact zone; mixing occurs gradually across the interface rather than instantaneous blending.
level_4: All conditions for score 3 are satisfied; additionally: the blue-dyed saltwater remains predominantly below the freshwater throughout the observed mixing zone; no upward intrusion of blue water into the upper freshwater layer; no downward sinking of freshwater into the blue layer; interface shape is stable and consistent with gravity-driven two-layer flow (no inversion or oscillation).
level_5: All conditions for score 4 are satisfied; additionally: the interface exhibits characteristic estuarine density-driven behavior: blue saltwater undercuts the freshwater, forming a sharp lower layer that advances upstream relative to the freshwater inflow direction; the mixing zone shows fine-scale interleaving or small-scale instabilities (e.g., subtle undulations or gentle scallops) consistent with Kelvin-Helmholtz or gravitational shear, not turbulent chaos or artificial diffusion.

Spatiotemporal Consistency rubric:
level_1: Flume or streams teleport between frames; abrupt discontinuity in stream position, width, or color between consecutive frames; sudden appearance/disappearance of either stream; lighting changes inconsistently (e.g., shadow jumps, specular highlight vanishes then reappears); structural break in flume wall or base.
level_2: Minor but repeated spatial jitter in stream edges or flume boundaries; intermittent flicker in stream visibility (e.g., one frame shows full stream, next frame shows partial occlusion with no cause); inconsistent light direction causing contradictory highlights/shadows on same surface across frames; flume perspective shifts (e.g., zoom, pan, or rotation) without justification.
level_3: Flume remains fixed in frame with consistent geometry and orientation; both streams maintain continuous presence and spatial coherence from entry to mixing zone; no teleportation, flicker, or structural break; lighting remains globally consistent (same dominant direction and diffuse character); motion of water surfaces is smooth and physically plausible (no strobing, freezing, or unnatural acceleration).
level_4: All conditions for score 3 are satisfied; additionally: stream widths remain proportionally stable along their path (no artificial narrowing/widening unrelated to flow dynamics); interface region maintains consistent lateral alignment (no drifting left/right relative to flume centerline); no frame-level aliasing artifacts (e.g., moiré on flume grid, zipper edges on stream boundaries).
level_5: All conditions for score 4 are satisfied; additionally: inter-frame motion is perfectly fluid with no temporal stutter or micro-judder; stream surfaces exhibit natural, low-amplitude undulation consistent with laminar inflow; flume walls show uniform reflectivity and no lens distortion; no inter-frame misregistration of foreground/background layers (e.g., parallax shift between flume glass and internal flow).