Do wind vent holes in banners make a difference? We used a wind tunnel to find out

Do the holes in the banner carried by these Vietnam veterans during an Anzac Day parade in Canberra make any difference?

Do the holes in the banner carried by these Vietnam veterans during an Anzac Day parade in Canberra make any difference? AAP Image/Alan Porritt

Matthew Mason, The University of Queensland and Jonathan Roberts, Queensland University of Technology

The next time you see a banner hung across a street or from a bridge, or hoisted as part of a street march, protest or demonstration, take a closer look. You may see that the banner has holes or slits cut into it.

But why would someone cut holes into a perfectly good banner?

These are so-called “wind vents”, and for some reason people have been mutilating their banners with these holes in the belief that their presence will significantly reduce the wind loading on the banner.

But does a banner with holes or slits really have an easier time in the wind than an equivalent banner that is hole free?

History and legislation

It is not known when people started to cut holes into their banners. There is very little written about the practice, and much of the knowledge appears to come via word of mouth or has been transferred from other wind related domains.

What is obvious from the websites of the world’s sign and banner makers is that they are frustrated with having to cut holes into their lovingly-made creations.

Some banner makers simply refuse, and tell their customers that if they want holes, then they can cut them themselves.

The apparent importance of banner wind vents has led some local governments around the world to make them mandatory for banners installed in certain locations. No vent holes, no banner allowed!

The regulations of the Brisbane City Council, in Queensland, Australia, state that for banners to be installed on the city’s iconic Story Bridge, they “must be provided with wind vent holes” and that “wind holes (vents) need to be spaced at approx. 3m intervals”.

Brisbane City Council’s Story Bridge banner design guide indicating location of ‘wind vent holes’.
Brisbane City Council design guide

The small town of Springville, Utah, USA, states in its regulations that at least 20% of the area of the banner must be made up of holes. It suggests “half moon shaped vents 4-6 inches wide and facing down throughout the banner”.

Understanding the aerodynamics

To understand what, if anything, wind vents do for our banners, we need to visit the work of aerodynamics specialists.

In 1956, B. G. de Bray, an aerodynamics expert at the UK’s Royal Aircraft Establishment, performed a series of wind tunnel tests to show how flat plates with holes in them performed in a moving air stream. He was interested in how plates could be used for airbrakes on aircraft as they land.

His experiments showed that perforations (holes) make the air flow more stable but that there was “only a comparatively small reduction in drag coefficient”. He shows a graph recording the relationship between the area of the holes and the change in drag coefficient of a flat plate. The graph indicates that making 20% of a banner’s area holes will reduce the drag by around 5% in a wind of 150km/h.

These figures are taken from de Bray’s 1956 work on wind tunnel testing of flat plates with holes and how drag relates to hole area in a 150km/h wind. Note that CD designates the drag coefficient, which is a normalised way of representing force that accounts for plate size (or in our case the banner) and wind speed. Doing this allows the wind tunnel data to be scaled to full-size.

When we consider de Bray’s other finding – that holes do make the air flow more stable – we can look at a common example of this in action in round parachutes.

Billowing structures that fill with air on the windward side, such as round parachutes, become unstable when there are no holes in the structure. The air tends to spill almost randomly from the structure’s edge. This makes the structure flap around in the wind in a seemingly random manner.

This was discovered in the early days of parachute development. In the late 1700s, a number of parachute developers were killed due to accidents relating to their unstable and oscillating chutes.

In 1804, Frenchman Joseph Lelandes invented the apex vent, a hole in the top of the parachute. This appeared to solve the problem of stability but did not appear to reduce the drag, ideal for parachuting where you need the drag.

Since then there have been many studies showing the benefits of holes in round parachutes. One group even found during their experiments that vent holes in round parachutes slightly increase the drag on the chute while making it more stable.

Wind tunnel tests

Following in de Bray’s footsteps, we decided to turn to wind tunnel experiments to assess just how much impact those holes had on wind forces.

We conducted a series of simple experiments where we put scaled versions of banners in a wind tunnel and measured the wind forces. We did this for a range of wind speeds and number of vents (holes). We then measured how the forces changed from test to test.

We performed experiments where vents were rectangular holes cut in the fabric and others where the vents were rectangular holes cut on three sides and allowed to hinge at the top (flaps).

A test banner with 7% of its area made of holes in the wind tunnel.
Author supplied
As for above, but showing a banner with 7% porosity and hinged flaps.
Author supplied

Experimental wind speeds tested ranged from approximately 25km/h to 100km/h and the range of vent hole area to total banner area ratios (porosity) assessed was from zero (no holes in the banner) to approximately 20%, which coincides with the Springville regulations and makes a pretty holy banner.

A plot showing drag on the banner versus porosity of the banner for the 100km/h tests over the range of banner porosities. The vertical axis shows the drag coefficient (CD) ratio, which is the wind force measured on the porous banner divided by the wind force on the solid banner. A porosity of 0.1 is 10% holes/vents/flaps.
Author supplied

A value of 1 in the figure (above) would indicate that the vents have done nothing and a value of 0.9 would suggest there has been a 10% reduction in load.

It is clear that wind vents do reduce the wind load on a banner, but as de Bray showed, the reduction in load is relatively small until porosity becomes large.

The reduction in drag force is greater for holes and hinged flaps than found by de Bray (and others) for uniformly perforated plates or fabrics.

The wind speed makes a difference. At low wind speeds the presence of vents can actually increase the wind load on a banner, which in our test was found to be up to 5%.

In general though, force coefficients decreased as wind speeds increase. This was particularly the case for the banners with flaps, where these vents became more open as the wind speed increased.

So the type of vent makes a big difference. Banners with holes rather than hinged flaps experienced lower wind loads. Both of these vent types experience lower loads than on uniformly perforated plates, which perform similarly to porous mesh fabrics.

With these points in mind, we return to the Brisbane City Council’s regulations for placing banners on the Storey Bridge. It is now possible to calculate the effect of their prescribed wind vents.

If we assume that they would like holes, and the maximum size of a banner is 18m wide by 0.9m high, then our best guess estimate is a semi-circular hole radius of 25cm noting also that five wind holes are required. We calculate that at most, 3% of the banner will be holes.

Interpolating our figure this would give us a 2% reduction in wind load. A sign of 98% the area of the maximum would be 18m wide and 0.88m high and would only require you to trim 2cm off the bottom of the sign to create a sign of equivalent drag to the one with five holes in it! It hardly seems worth the effort.

The verdict

The science shows us that flat structures behave one way, and billowing air-filled structures behave a different way. It seems that our legislators have been confused and applied results from parachutes to flat banners.

If you have a banner tied in such a way that it will remain relatively flat in the wind, then it seems that the benefits of putting in vents are minimal unless you make your banner into Swiss cheese.

You are simply better off making a slightly small banner to achieve the same reduction in load.

The Conversation

Matthew Mason, Lecturer in Civil Engineering, The University of Queensland and Jonathan Roberts, Professor in Robotics, Queensland University of Technology

This article was originally published on The Conversation. Read the original article.

Australia’s first robotic help in a hip replacement operation

The surgeon and the robotic arm will work together on a hip replacement.

The surgeon and the robotic arm will work together on a hip replacement. Stryker, Author provided

Ross Crawford, Queensland University of Technology; Anjali Jaiprakash, Queensland University of Technology, and Jonathan Roberts, Queensland University of Technology

The first robotically assisted hip replacement operation in Australia is due to be performed today on a patient in Brisbane.

A total hip replacement (THR) is one of the most successful operations that surgeons perform, with more than 43,000 carried out last year in Australia alone.

The robot technology to help in such operations has been used for some years in the US but has only recently reached Australia.

But if the operations are so popular and successful, why let a robot in on the surgery?

The hip opp

A hip replacement involves an incision to expose the hip joint and the placement of an acetabular component (the cup) and a femoral component (the stem). A head is then placed on the stem and a ball and socket joint is created that is the patient’s new hip.

A typical ball and socket artificial hip replacement.
Ross Crawford, Author provided

Though very successful, the operation can be quite challenging to perform in certain patients such as the very overweight and those with complex deformities due to childhood diseases or trauma. There is also a learning process for the surgeon in performing a hip replacement and it is hoped this can be shortened by using robotic technology.

Accurate positioning of the components of a hip replacement is important. Having the cup and stem in the correct position can decrease the chance of complications such as dislocation, where the head comes out of the cup. Making sure the joint stem is located in a way to ensure optimal leg length may also lead to improved function of the new hip.

Currently, surgeons rely on their experience and judgement to correctly place the components of a hip replacement. Many studies have shown that even experienced surgeons can have difficulty in reliably and accurately placing the cup in the correct orientation. They sometimes find placement of the stem challenging too.

This is where a robot can help.

The robot surgeon

Up until now, the Australian experience of robotic orthopaedic surgery has been limited to partial knee replacements. The first was carried out in April last year, and since then more than 280 of these procedures have been performed.

The first robotically assisted total hip replacement operation will take place today at Brisbane’s Holy Spirit Northside Hospital, and it’s likely such procedures will quickly become just as popular as the knee operations.

The Stryker Mako advanced robotic arm that helps with the surgery.
Stryker, Author provided

So what is different with a robotic total hip replacement and where does the robot help?

The MAKO robotic system is a carefully controlled robotic arm that aids surgeons in placement of the components of a total hip replacement. It makes the operation more accurate and safer for surgeons, regardless of their experience.

The main difference from a patient’s point of view is that a pre-operative CT scan is needed to plan the procedure. Traditionally, surgeon relied purely on an X-ray to plan a total hip replacement.

When performed by a robot, planning for the procedure is done by specialist engineers in collaboration with the surgeon. The engineer and surgeon work together to determine the optimal position for the components and they create a plan.

The plan places the cup in the correct orientation to match the patient’s anatomy and the stem is also sized to fit the patient’s femur. The aim is to accurately restore the patient’s hip anatomy, particularly leg length.

Once the surgery begins, the surgeon exposes the hip joint in the usual way. Trackers are placed on the pelvis and on the femur allowing the robot to register these bones.

The trackers are attached to the bones using small posts with a screw thread on the tip. A series of points on the patient’s pelvis and femur are then registered and the robot creates a 3D representation that matches the CT scan.

Once the robot understands the geometry, it is able to follow any movement of the patient by the signal transmitted by the trackers fixed to the bones.

A cutting tool called reamer – somewhat like a powered round cheese grater – is attached to the robot and is used to prepare the bone to accept the cup. The surgeon holds the reamer but the robot constrains it and will not let the surgeon remove bone beyond the planned amount.

This will prevent any accidental damage to the bone and make sure the reaming can only occur as planned. Human error is removed from the preparation.

After reaming is finished, the cup is grasped by the robot and the robot sets the correct positioning. The surgeon then hammers the cup into the correct position in the pelvis.

They are able to monitor the position of the implant on the computer screen as it is “seated”. The cup cannot be driven in too far, as the robot constrains where the cup can be placed, as with the reamer.

Next the surgeon places a broach in the femur to prepare a cavity for the femoral component (stem). The broach can be tracked by the robot to make sure it is placed in the correct orientation and the patient’s legs are at the planned length.

Once happy, the surgeon cements the stem into where the broach was positioned, places a head on the femur and puts the head into the cup.

Who’s in charge?

Though the robot is constraining the surgeon to execute the plan, the surgeon remains in charge at all times. The surgeon continues to carry all responsibility for the success of the operation and any complications.

This first step of robotically assisted total hip replacement is relatively easy. The robotic technology (robotics, navigation and haptics) being used is very mature.

But as we are seeing in many industries, the capability of robotics is expanding rapidly. It will not be long before the technology is advanced enough to take over far more of the operation from the human surgeon.

Then the big ethical questions will arise. Even now orthopaedic robots are being limited in what they can do because the step to autonomous surgery is currently a step too far.

Like driverless cars, the questions of liability and trust continue to be aired when discussing robotic-surgery or health care.

But also like driverless cars, robotic surgeons do not have to be perfect. They just have to be better than humans.

The Conversation

Ross Crawford, Professor of Orthopaedic Research, Queensland University of Technology; Anjali Jaiprakash, Post-Doctoral Research Fellow, Medical Robotics, Queensland University of Technology, and Jonathan Roberts, Professor in Robotics, Queensland University of Technology

This article was originally published on The Conversation. Read the original article.

New relaxed drone regulations will help the industry take off

CASA makes it easier for low risk flying of drones. Flickr/Richard Thorek, CC BY-NC-SA

Reece Clothier, RMIT University and Jonathan Roberts, Queensland University of Technology

The Australian drone industry is set for a shake up following the announcement of a long-awaited relaxation of regulations on their operation.

Australia’s Civil Aviation Safety Authority (CASA) says the amended regulations will come into effect in late September 2016, and with them comes the introduction of new categories of what are known as remotely piloted aircraft systems (RPAS).

The regulations define new low-risk commercial RPAS operations, which will allow operators of sub-2kg craft to fly without the need for an approval or licence.

A drone must be operated in daytime and within visual line of sight of the remote pilot to be classified as low risk. It must not be flown over populous areas and must be kept at least 30 metres from other people.

The drone cannot be flown greater than 130m above ground and it must not be flown within 5.5km of a controlled airport.

Commercial operators in this new category will have to register their operations with CASA on a yet-to-be live website.

Relaxed regulations will also apply to private owners of RPAS of up to 150kg. This is provided they only fly their drone over their private property and they do not operate their aircraft for direct commercial reward.

Why the change?

In 2002, CASA was the first in the world to regulate the operation of drones.

The regulations, contained in Part 101 of the Civil Aviation Safety Regulation (CASR 1998), were long considered ground breaking. Much of the success of the Australian unmanned aircraft industry is owed to the flexible approach outlined in the regulations.

In 2007, there were fewer than 25 certified drone operators in Australia. By March 30, 2016, this number had grown to 500, with most operating small multi-rotor RPAS.

But with this rapid growth came the increasing need for regulatory reform. CASA recognised that the regulations needed to keep pace with increasingly capable technology, and the changing operational needs of the sector.

It also realised that processing an ever increasing number of regulatory applications was not sustainable.

Welcome news

The new changes will significantly reshape the drone industry.

Operators already licensed by CASA are expected to face increased competition from the new sub-2kg RPAS operators. These new operators will be able to provide equivalent aerial photography and inspection services without the same regulatory overhead.

Similarly, there will be an increase in the number of end-users choosing to own and operate their own internal RPAS capability instead of contracting existing RPAS service providers. Examples include the use of small inspection drones on building sites and the use of drones by tactical police units to assist them in hostage situations.

But it is not all doom and gloom for the current licensed RPAS operators. The standard operating conditions applicable to the new low-risk categories are restrictive.

Larger and more reliable drones will still be needed to carry bulky and more expensive payloads such as laser scanners, and hyper-spectral and cinema-quality cameras. These drones will still need to be operated by licensed operators.

Approval is still required for first person view (FPV) outdoor flying operations, where the remote pilot flies by means of a camera mounted on board the drone.

Similarly, autonomous drones, which operate without any input from a pilot, also require CASA approval on a case-by-case basis.

A large drone that will still require licensed operators for commercial use.
Stefan Hrabar/CSIRO/UAV Challenge

Research and educational institutions, such as universities, are also expected to benefit from the new categories, provided they operate their aircraft over their own property and in accordance with all other operational restrictions.

Previously, these institutions were subject to the same licensing requirements as commercial operators.

Hobby users

The amended regulations do not address concerns posed by the rapidly growing number of hobby drone users.

Regulations applicable to hobby or recreational users are contained in CASR 1998 Part 101.G, which is the subject of a separate CASA regulatory reform project.

There is growing concern over the risks hobby users pose to other aircraft and to members of the public. Some of these hobby users are not aware of the potential danger their drone may pose.

There have been numerous near misses of small drones with passenger aircraft in recent years. As the rate of these incidents increases, there is real concern that a drone will eventually be ingested into an aircraft engine causing catastrophic damage – or worse, an airline crash.

Others are well aware of the dangers their drones may pose to the public but they are deliberately mischievous anyway.

Education remains the only effective tool, with CASA leading a campaign to educate hobby users on the safe operation of their aircraft and the regulations that apply to them.

Without doubt, the release of the amended regulations will mark a significant milestone in the history of the Australian drone industry. They will help to sustain the safe and viable growth of the sector.

But the devil may still lie in the detail, of course, with the accompanying manual of standards yet to be released by CASA. The manual will contain more detailed requirements including those for remote pilot licences, flights in controlled airspace, and flights beyond visual line of sight of the pilot.

CASA’s exact interpretation of “Aerial Work” and “Commercial Reward” also remain unclear.

The Conversation

Reece Clothier, Senior Lecturer, RMIT University and Jonathan Roberts, Professor in Robotics, Queensland University of Technology

This article was originally published on The Conversation. Read the original article.