Flow Control &
Experimental Turbulence Laboratory

Airframe Noise

Aircraft noise emission and how it affects communities is an remain important concern in the aviation industry. Because recent technological innovations have led to quieter engines, the noise due to the airflow around the surfaces of the aircraft, known as airframe noise, is playing an increasingly important role in the overall noise emissions. For small to mid-size commercial aircraft, the dominant sources of airframe noise during the approach and landing phases are landing gears and high-lift devices, such as slats and flaps, on the wings. For these reasons, we have been working on a number of projects related to understanding and mitigating the noises sources from both landing gears and high-lift devices. For these studies, models are tested in our open-jet anechoic wind tunnel, which allows for accurate measurements of noise emissions to be taken. Through combined aeroacoustic microphone measurements and PIV measurements of the flow field, we are finding new insights that are leading to quieter design and contributing to the improvement of computational aeroacoustic (CAA) tools used by engineers.

Nonparallel Cylinders in Tandem Arrangement

In order to reduce the airframe noise emission from a landing gear, it is important to identify its sources and understand the fluid mechanics phenomena that produce them. We focus on the nonparallel tandem cylinder configuration, which is a good representation of a common geometry of a landing gear but is largely unexplored. Experiments in this project are performed in the anechoic wind tunnel - an open loop, open jet facility and the general arrangement of our model can be seen in the photo below. A wide range of different Re numbers (20,000 – 140,000) and inclination angles (0 – 45 degrees) are being tested.

We perform PIV experiments with simultaneous pressure measurements to obtain a clear picture of the flow field and specifically the vortex shedding behavior. In particular, changes in the shedding frequency over the cylinder(s) span allow us to deduce specific patterns of the vortex shedding which can have a significant effect on the aerodynamic loads and noise emissions.

Our results are providing insight into how the different flow regimes such as reattachment of the separated shear layers from the upstream cylinder on the downstream cylinder, as well as independent vortex shedding from each cylinder (co-shedding) behave in this type of configuration.

Blunt Trailing Edge Wake Control

The blunt trailing edge profiled body is a bluff body where the dominant feature of the near wake is a vortex street, causing relatively high drag compared to a streamlined body. Blunt trailing edge airfoils and landing gear are common examples of bluff bodies in aviation.

We are investigating an active control method to reduce the drag and the periodic velocity fluctuations in the wake of a blunt trailing edge profiled body by attenuating the vortex street. Control of the vortex street is accomplished through an array of synthetic jet actuators distributed along span on the upper and lower surfaces near the trailing edge of the body. The synthetic jets generate pairs of streamwise vortices in the boundary layer of the body, resulting in a distributed three-dimensional disturbance to the vortex street. This reorients the wake vorticity and forms streamwise vortex loops along the span, which weakens the vortex street by up to 40% and reduces the drag by 30%.

We have also found that the efficiency of this forcing strategy can be improved using amplitude modulation at frequencies comparable to the vortex shedding frequency. Furthermore, as this forcing strategy is fundamentally "three-dimensional", we are also interested in better understanding the natural three-dimensional structure of the near wake and how it varies as the flow transitions from laminar to turbulent. In particular, we are investigating the characteristics of large-scale wake patterns that emerge from oblique vortex shedding, as well as relatively shorter wavelength coherent structures that originate from a secondary instability.

The Round Turbulent Jet

Turbulent round jets are present in many engineering applications and is also of fundamental scientific interest. In particular are the turbulent entrainment process and the mechanisms that control the transport of mass, momentum, and scalars from a turbulent region of a fluid to a non-turbulent region. We attempt to address some of the questions about the role of coherent structures in the entrainment of axisymmetric turbulent jets with the aid of Particle Image Velocimetry (PIV), Laser Induced Fluorescence (LIF), and Hot-Wire anemometry.

Using two-point Hot-Wire measurements, we assess eddy structures (ESs) and very-large-scale motions (VLSMs) in axisymmetric turbulent jets. A spectral analysis is applied to synchronized axial velocity datasets with varying separations between the velocity sensors in the azimuthal and radial directions. Using these results, we show that a hierarchy of eddy structures are embedded in the jet.

To study the effect of boundary condition on the dominating length scales in the turbulent entrainment of axisymmetric turbulent jet, we conduct a campaign of simultaneous PIV and LIF measurements at several axial stations downstream of the jet nozzle. We investigate the scaling of the mass flux and entrainment velocity across the turbulent/non-turbulent interface (TNTI) in the near- and intermediate-field of an axisymmetric jet at high Reynolds number. Time-resolved, simultaneous PIV and LIF are used to identify and track the TNTI, and directly measure the local entrainment velocity along it.

Laminar Separation Bubbles

Flow separation is a common occurrence at low Reynolds numbers due to adverse pressure gradients. After flow separation, laminar to turbulent flow transition may occur, which can re-energize the boundary layer sufficiently to reattach the flow. The region between separation and reattachment is called a laminar separation bubble (LSB). With the use and demand of unmanned aerial vehicles increasing, it has become increasingly important to account for these unique flow phenomena, as UAVs are designed and tailored to their respective flight regimes. Modern experimental techniques such as oil film interferometry and particle image velocimetry have enabled us with an enhanced understanding of LCBs. This experimental work focuses on determining the behaviour of an LSB with various freestream turbulence intensities and other flow parameters.

High Performance Sports Aerodynamics

We have worked on a number of sports aerodynamics projects. The goals of these projects have been to apply our extensive knowledge of aerodynamics and flow control to produce novel low drag configurations for a number of applications. One notable application dealt with the development of an ultra-performance triathlon bicycle that was designed by our industrial partner, KQS Inc, and manufactured by DiamondBack – the DiamondBack Andean. Other projects have included bicycle wheels and speed suits.

Bio-inspired Aerodynamics

Through collaborations with colleagues at the University of British Columbia and University of Manchester, we have considered the aerodynamics of avian flight and the role of wing morphing on bird flight stability and efficiency. These multi-disciplinary projects combine biomechanics, modeling, aerodynamics and flight dynamics to better understand avian flight, in particular gliding.

Synthetic Jet Forcing of a Turbulent Boundary Layer for Skin-Friction Drag Reduction

All streamlined bodies travelling through a fluid develop a boundary layer where the velocity transitions from zero at the surface (no-slip condition) to the speed of the object relative to the fluid at some distance (the boundary layer thickness) away from the surface of the body. This boundary layer takes energy away from the mean flow, causing skin-friction drag. At high velocity and/or in low viscosity flows (i.e. at higher Reynolds number), which is the case for many applications of engineering interest, this boundary layer transitions from smooth laminar flow to random turbulent flow. In a turbulent boundary layer streamwise-aligned vortical structures (“eddies”) pull higher-speed fluid from farther out in the boundary layer closer to the surface of the body, increasing the skin-friction drag relative to a laminar boundary layer. This turbulent skin-friction drag can make up a significant proportion of the total drag on a body.

As an example, for a typical commercial airliner the skin-friction drag, due to primarily turbulent boundary layers, comprises about 50% of the total drag on the aircraft. Thus, it makes both economical and environmental sense to find ways to reduce the turbulent skin friction drag. Many researchers are focusing on directly targeting the eddies close to the surface to reduce drag. The problem with this is that these eddies are small and become increasingly smaller as the Reynolds number is increased, which makes instrumentation difficult at flight-scale Reynolds numbers. Furthermore, at high Reynolds number, larger eddies further out in the boundary layer become increasingly more energetic relative to the smaller eddies closer to the surface.

Our approach is indirect and relies on the fact that these larger eddies have been shown to have a significant impact on the smaller drag-producing ones close to the surface. We use synthetic jet actuators, which alternately ingest and expel fluid (forming a synthetic jet) at the surface of the body to interact with the large eddies. Current research is focused on understanding the interaction between eddies produced by the synthetic jets and the large eddies present in the turbulent boundary layer to uncover a mechanism that can be leveraged for drag reduction. Synthetic jet forcing has been shown to reduce the strength of the large eddies, but the impact on the skin-friction drag is not clear at present. There are likely competing effects at play: increase in drag due to the redistribution of fluid from the eddies produced by the synthetic jets, as well as decreases in drag due to the breakup of large eddies.