Wind tunnel testing: when the cost is unavoidable

Wind tunnel test cost planning decisions are mostly decisions about sequencing. The test itself is a fixed piece of work: a scale model of the scheme and its surroundings, a boundary layer tunnel, a set of measurement positions at pedestrian level, and a report comparing the results against comfort and safety criteria. What makes it expensive is doing it twice, which is what happens when the massing changes after the model has been built.

On a tall building scheme the test is usually unavoidable. On a mid-rise scheme it often is not, and a competent desktop assessment can close the issue for a fraction of the cost.

Desktop assessment as the first step

A qualitative wind microclimate assessment is written by an engineer who knows how flow behaves around buildings, using local wind statistics from the nearest long-record weather station and the geometry of the scheme and its context. It identifies the mechanisms likely to cause problems: downwash on tall windward facades, corner acceleration, channelling between buildings, through-flow at undercrofts and gaps, and the effect of a podium edge on a terrace above.

A Wind Microclimate Assessment at this level can conclude one of three things. That conditions will be acceptable for the intended uses of each external space, with reasoning. That conditions are likely to be acceptable subject to specified mitigation such as a canopy at the entrance or a screen at a terrace corner. Or that the scheme's height and form mean the outcome cannot be predicted with confidence and quantitative assessment is needed.

That third conclusion is the one that triggers the cost, and an experienced assessor will reach it early rather than after three rounds of correspondence. Councils expect a suitably qualified author and professional review is strongly advised for this report type.

When a council insists on physical testing

Four triggers come up repeatedly.

• The scheme is a tall building as defined locally, or is appreciably taller than its surroundings. London Plan policy on tall buildings requires wind microclimate to be addressed, and boroughs commonly read that as requiring quantitative assessment.
• The scheme provides external amenity space at height: roof terraces, podium gardens, balconies intended for use.
• The scheme sits next to a sensitive receptor: a bus stop, a station entrance, a cycle route, a market square, an outdoor seating area.
• A desktop assessment has identified potential exceedance of the safety criterion, which is treated differently from comfort because it concerns people being blown off balance rather than being uncomfortable.

The safety point deserves emphasis. Comfort criteria are a matter of planning balance, so a windy corner that is acceptable for brisk walking but not for sitting can be accepted with the use relabelled. A predicted exceedance of the distress or safety threshold is not balanced away. It has to be designed out, and that is why authorities insist on quantitative evidence where there is any prospect of it.

Model making and tunnel time

The cost of a physical test breaks into four parts, and understanding them lets you control the quote.

Configurations are the item that multiplies. A typical planning study tests the existing situation, the proposed scheme, the proposed scheme with mitigation, and a cumulative case including consented neighbouring schemes. Each of those is tunnel time, and each additional seasonal or variant case adds more.

Laboratories in established city centres often hold a context model already built, which removes a meaningful share of the cost. Ask the question before accepting a quote that assumes a new context model.

Computational modelling as an alternative

Computational fluid dynamics has two uses on these schemes and they are different.

As a design tool it is excellent. Running massing variants, testing a canopy depth, moving a screen, checking whether a recessed entrance solves a downwash problem: all of that is faster and less costly computationally than in a tunnel, and it is where the design actually gets fixed.

As planning evidence it is accepted by some authorities and resisted by others. Where it is used for submission, it needs to follow accepted modelling conventions on domain size, mesh resolution, turbulence modelling and the treatment of the atmospheric boundary layer, and the report has to document those choices so a reviewer can check them. A computational study that does not explain its assumptions will be challenged, and the challenge is usually fair.

The pragmatic sequence on a tall building is computational modelling to develop and test the design, then a physical tunnel test of the resolved scheme for the submission. That costs both, and it still costs less than tunnel-testing four massing options.

Reruns after a massing change

This is the expensive mistake. A physical model is built to the massing as designed, and a change to height, setback, parapet form or podium edge invalidates the results for the affected area. In practice a meaningful massing change means a new model and new tunnel time.

Three ways the rerun happens:

1. The design develops after testing, because the test was commissioned to meet a validation deadline rather than at design freeze.
2. The authority asks for a lower or reshaped upper storey during determination, which is common on tall buildings.
3. A neighbouring consent is granted during determination, changing the cumulative case.

The first is avoidable and the second is not. So build one rerun into the budget on any scheme where the massing is likely to be negotiated, and ask the laboratory at the outset what a partial rerun would cost, because a change affecting one elevation does not always require everything to be redone.

The third is worth tracking actively. Monitor neighbouring applications through the determination period, because a cumulative case that ignores a consent granted two months earlier will be criticised.

Timing against the design programme

Allow weeks rather than days. Model fabrication, tunnel availability and reporting each take time, and tunnel slots in the busier laboratories are booked ahead.

Three programme points to settle when the work is commissioned. The lead time from instruction to model completion. The earliest available tunnel slot, confirmed rather than estimated. And the reporting period, including a draft for comment before the final version, because the interpretation against comfort criteria usually needs a conversation about what each external space is for.

Coordinate with the other environmental studies. The daylight and sunlight work, which runs on the same massing, should be commissioned at the same design freeze so that a massing change triggers one coordinated set of reruns rather than two sequential ones. The relationship between those studies and the massing is covered in the guide to the Daylight and Sunlight Assessment.

Budgeting on a tall building scheme

Treat the wind work as a two-stage line in the fee budget rather than a single item.

Stage one is the desktop assessment, commissioned early, whose job is to tell you whether stage two is needed and to shape the massing before it is fixed. On many mid-rise schemes stage one is the whole of the work.

Stage two is the quantitative study: computational modelling during design development, then a physical test of the resolved scheme where the authority requires it, with a contingency for one rerun. Add the cost of mitigation itself, which is real money in canopies, screens, balustrade infills and planting, and which lands in the construction budget rather than the fee budget.

One practical caution about mitigation planting. Trees take years to reach the size assumed in the model, and an assessment that relies on mature planting to achieve comfort is vulnerable, as is one that models trees in leaf when the windiest months are in winter. Say which condition was modelled.

For specialist technical reports, qualified professional input may be required, and the monitoring and survey periods behind other technical studies follow a similar pattern, as set out in the guide to budgeting a noise survey and its monitoring period.

You can compare what the supporting planning documents cost to draft on the PlanScope pricing page, and see the full list of report types in the report index.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.