Stir it up: The impact of building induced turbulence

11/08/2006

Stir it up: The impact of building induced turbulence

Onderstaand artikel van Robert Brons is overgenomen uit IFALPA news, juli 2006.
Robert is lid van de Aircraft Design and Operation Committee en vertegenwoordigt IFALPA bij de Committee on Aviation Environmental Protection (CAEP).

For a number of years the pilot community has voiced concerns about the reliability and accu-racy of reported wind for takeoff and landing. Of course we are dealing with a volatile and unstable atmosphere and as a result the wind is constantly changing in strength and direction.

The reported wind is, at best, a picture of the current wind conditions based on statistical data rather than an accurate picture of what is experienced during the flare, landing or rollout. As anyone with even a few hours flight experience has found out, building-induced wind disturbances will tend to magnify the natural gustiness and instability of the airflow. While even the newest of student pilots knows that buildings induce turbulence, the question has always been just how much turbulence do they cause? Of course, without quantifiable data as evidence it has been difficult to limit the proliferation of new buildings around and near runways.

…but that may be about to change

The advent of computational fluid dynamics software, together with sophisticated mathematical wind models which can display a graphic of the area of disturbed wind on the downwind side of a building and accurately describe three dimensional wind patterns in the lee of an obstruction, is a significant step forward. Taking the data from these resources and applying them in an experimental simulator that is able to incorporate more dynamic variables has produced a more realistic test scenario.
In 2004, the Dutch National Aerospace Laboratories (NLR), in association with the Dutch Air Line Pilots Association (VNV), began researching the impact of various obstacles situated along a runway on the fly ability of takeoffs and landings. The panel used a new wind model developed by the NLR to represent wind patterns around buildings and other obstacles. Then a theoretical safety study was carried out and followed by ‘offline’ simulation runs in the F100 at the University of Delft and the B-747 at the NLR. In addition, pilot flown, real time simulator experiments were conducted on the Grace simulator to validate the off line runs as well as introduce human performance factors into the test
regime.

Initial results

Wind disturbance induced by structures, or indeed terrain, can be divided into three components; windshear, turbulence and generated vortices. Studies indicate that the vortices are carrying the wake further downwind from the obstacle (up to 50 times the height of the building). Early results indicate that unsurprisingly, aircraft controllability is most affected by windshear and turbulence and this impact is increased by the terrain surrounding the buildings and runway. What is new information are the rates of gusts and the maximum gust values, which are significantly changed by the presence of buildings and other obstacles.
At Amsterdam, wind conditions have led to unstable approaches and hard landings, or have resulted in late go-arounds. The availability of high quality modelling and simulation has highlighted these elements which have been obscured in the past as far as hard data is concerned. These results clearly indicate the need for criteria to limit building plans near runways. These limits and their definition are being studied by the working group and it expects to produce preliminary codes in the near future.