BPS5234A Chap.9 Flow Around Bluff Bodies and Single Buildings
Flow Around Bluff Bodies and Single Buildings
Why bluff bodies come first
Buildings are bluff bodies: sharp-edged shapes around which the flow separates instead of following the surface. The first academic working examples study that family in increasing complexity. A two-dimensional square cylinder introduces laminar flow, wake formation, vortex shedding, drag and lift coefficients and the Strouhal number.
A heated cube on a plate adds three-dimensional structures and heat transfer.
A single building in an atmospheric wind turns the same physics into wind loads, ventilation potential and comfort.
The flow structures to recognise
Around a cube or building the approaching flow stagnates on the windward face, separates at the roof and side edges, rolls into a horseshoe vortex around the base, and leaves a low-pressure, highly turbulent wake.
On the heated cube these structures shape heat transfer: high Nusselt numbers at stagnation and the top leading edge, lower values on the upper front and sides, and the lowest in the wake.
The numbers each example reports
The cylinder reports a Strouhal number from the shedding frequency; the course advice is to run long enough, with a 0.002 s time step to 16 s, to see full shedding.
The cube reports Nusselt numbers and a drag coefficient. The building reports pressure coefficients compared with classic wind-tunnel data.
Domain and mesh
The decks give domain rules in multiples of the building height and ask for refinement at edges, in boundary layers and in the wake, with mesh independence tested properly.
What this chapter covers
- 01
Bluff-body separation, wakes and vortex shedding
- 02
Reynolds, Nusselt and Prandtl numbers
- 03
Strouhal number from a lift signal
- 04
Horseshoe vortex and heat-transfer zones on a cube
- 05
Pressure coefficients on an isolated building
- 06
Domain rules and mesh independence
Worked example · free
Nusselt number on a heated cube face
- 2Find the heat transfer coefficient from flux and temperature difference.
- 2Form the Nusselt number with the cube height.
- 1Interpret the value against the rear face.
Key terms
- Bluff Body
- A shape with sharp edges or bluntness around which the flow separates and forms a large wake.
- Strouhal Number
- The shedding frequency multiplied by a body size and divided by the flow speed.
- Nusselt Number
- The ratio of convective to conductive heat transfer at a surface, used to compare heat transfer across sizes.
- Horseshoe Vortex
- The vortex that forms where the approaching boundary layer rolls up in front of an obstacle and wraps around its base.
- Prandtl Number
- The ratio of momentum diffusivity to thermal diffusivity, about 0.71 for air.
- Separation Bubble
- A region of recirculating flow formed where the flow detaches from a surface, such as just behind a roof edge.
Flow Around Bluff Bodies and Single Buildings FAQ
Why is the square cylinder run as a transient case?
Vortex shedding is periodic, so a steady solution would average away the phenomenon the example is about. The course advises a long run with a 0.002 s time step and 20 iterations per step up to 16 s to capture full shedding and compute the Strouhal number.
Why does the front face of a heated cube transfer more heat than the rear?
The front sits in the stagnation region where the thermal boundary layer is thin, so heat leaves the surface efficiently. The rear sits in the recirculating wake where air is slow and already warmed, so the local Nusselt number is lower.
How large should the domain around a single building be?
The single-building deck gives at least five building heights upstream, fifteen downstream, five at each side and six in height. Too short a downstream length lets the wake reach the outlet and distorts pressures on the building.
What does a negative pressure coefficient on the roof mean for design?
It means suction: the surface pressure is below the free stream because the flow separates at the leading edge. The strongest suction near that edge governs cladding and fixing loads, and it also identifies a zone for possible outlet openings in ventilation design.
What should the report on a heated cube include?
Geometry and mesh schematics, boundary conditions and solver settings, vortex structures shown with streamlines and a Q-criterion field, surface temperatures on cube and plate, tables of Nusselt number, drag coefficient and flow parameters, and a comparison with experimental or LES correlations.
Assessment move
Sketch each benchmark from memory with its stagnation point, separation lines, horseshoe vortex and wake, then mark where each reported number is measured. A labelled sketch is the fastest preparation for the tutorial questions on flow physics.
