TisiCFD — Flow Lab

Obstacle

Reynolds number input

Fluid

Object size D

Flow speed U — 15.2 mm/s

Re = 0Re = 200Re = 400

Re = 100

Re ≈ 100 — Kármán vortex street

Angle of Attack (wing)

10°

0°5°10°15°25°

Inlet Profile

Visualisation

Smoke Streamlines

On — showing streamlines

410142024

Reference table: select any fluid to see the real velocity needed to achieve any given Re, across 4 object sizes. The Physical mode in the Flow tab lets you use these directly in the simulation.

🌬 Air 20°C

ρ = 1.204 kg/m³ν = 1.51×10⁻⁵ m²/s

🔥 Air 100°C

ρ = 0.946 kg/m³ν = 2.31×10⁻⁵ m²/s

💧 Water 20°C

ρ = 998 kg/m³ν = 1.00×10⁻⁶ m²/s

🛢 SAE 30 Oil

ρ = 875 kg/m³ν = 9.5×10⁻⁵ m²/s

🍯 Glycerin 20°C

ρ = 1261 kg/m³ν = 1.12×10⁻³ m²/s

☁ CO₂ 20°C

ρ = 1.839 kg/m³ν = 7.99×10⁻⁶ m²/s

🔬 Velocity table for Re = 1–400

Select a fluid above.

ReReynolds Number▶

Re = ρ U D / μ = U D / ν

Ratio of inertial to viscous forces. Low Re → viscosity dominates, flow is smooth. High Re → inertia wins, flow separates and becomes turbulent.

Key insight: same Re = same flow pattern regardless of scale. A 1cm marble in syrup can match a submarine in the ocean.

→ Use Physical mode — pick a fluid, change the speed slider, watch Re change.

🌀Kármán Vortex Street▶

St = f D / U ≈ 0.2 (Re 80–1000)

Above Re ≈ 47 the symmetric wake goes unstable. Vortices shed alternately from top and bottom — like a zipper of spinning eddies. The Strouhal number St gives the dimensionless shedding frequency.

In real life: power lines "sing" in wind, bridges can oscillate dangerously (see Tacoma Narrows).

→ Run Re 100–200 with vorticity view. Watch the red/blue alternating pattern shed downstream.

CᴅDrag Coefficient▶

Cᴅ = F_drag / (½ ρ U² A)

Dimensionless drag. Cylinder: Cᴅ ≈ 1.0. Wing at low AoA: Cᴅ < 0.05. Square (broadside): Cᴅ ≈ 2.0.

Streamlining works by delaying boundary layer separation, shrinking the low-pressure wake.

→ Compare Cᴅ stat between cylinder, wing, and diamond at the same Re.

ΔpPressure & Bernoulli▶

p + ½ρU² = const (along streamline)

Where fluid speeds up, pressure drops. High pressure at the front stagnation point, low pressure where flow accelerates around the sides, turbulent low-pressure wake behind.

This front-to-back pressure difference is what creates drag. The top-to-bottom difference on a wing creates lift.

→ Switch to Pressure view. See the red (high p) nose and blue (low p) wake.

✈Lift & Angle of Attack▶

Cₗ ≈ 2π α (thin-aerofoil, α in radians)

Lift is a pressure difference: upper surface has lower pressure than lower surface. Increasing AoA amplifies this — more lift, but also more drag.

Above critical AoA (~15°), the boundary layer peels off the upper surface. This is a stall — sudden lift loss.

→ Select Wing, push the angle of attack slider toward 20°. Watch the wake grow chaotic.

〰Boundary Layer & Separation▶

δ / D ∝ Re⁻¹/² (laminar BL thickness)

The thin slow-moving layer clinging to any surface. The no-slip condition means zero velocity at the wall. Higher Re → thinner boundary layer.

If pressure rises too fast along the surface, the layer runs out of momentum and separates from the wall — this creates the entire turbulent wake.

→ Try Boundary Layer inlet profile. See how the incoming shear changes where separation starts.

Configure the flow and click Run

Wind tunnel simulation · Re 1–400

Running simulation…

Chorin's Projection Method

2D incompressible Navier-Stokes on a Cartesian grid. Each step: (1) advance velocity with 2nd-order upwind advection + central-difference diffusion, (2) solve pressure Poisson ∇²p = ∇·u*/Δt, (3) correct velocity so ∇·u = 0. Adaptive CFL time-stepping for stability.

Flow Regimes

Re < 5: Creeping flow, fully attached.
Re 5–47: Steady recirculation bubble.
Re > 47: Kármán vortex shedding, St ≈ 0.2.
Re > 200: Irregular wake — 3D span-wise instabilities not captured.

Vorticity Field (ωz)

ωz = ∂v/∂x − ∂u/∂y. ■ Red = CCW rotation. ■ Blue = CW rotation. Obstacle in grey. White lines = smoke streamlines.