Flow Control &
Experimental Turbulence Laboratory

Research

The work in the FCET lab relies heavily on experimental data obtained in our wind tunnel facilities using a number of state of the art flow diagnostic methods, including hot-wire anemometry and laser based measurement tools (e.g. PIV). Computer simulations are also an important part of the work by complementing and enhancing the experimental investigations. An overview of ongoing projects is given below.

Current Projects

Unsteady Aerodynamics of Airfoils Aircraft Aeroacoustics Laminar Separation Bubbles Bio-inspired Aerodynamics Active Forcing of Turbulent Boundary Layers

Unsteady Aerodynamics of Airfoils

Unsteady Aerodynamics of Airfoils in Periodic Streamwise Gusts at Low Reynolds Numbers

Unmanned aerial vehicles (UAVs) are increasingly being used for a wide range of applications. These vehicles often operate at low speed in urban or mountainous terrain, where wind gusts are present and can have a fundamental impact on the aerodynamic performance of the aircraft. Our research is thus concerned with the experimental investigation of the unsteady aerodynamic behavior of airfoils, when they encounter periodic streamwise gusts at low Reynolds numbers.

The project is aimed at both: a) characterizing the unsteady response of an airfoil for a wide range of laminar and turbulent gusts and b) understanding the flow physics behind the observed trends in the airfoil response. A broad range of both laminar and turbulent streamwise gusts can be produced in the wind tunnel using a state-of-the-art active grid device, which modulates the freestream flow in the working section using a series of baffles. A multi-axis load cell is used to measure the unsteady loads on a SD7003 airfoil model, while the flow physics is investigated using smoke-flow visualization and particle image velocimetry (PIV).

SD7003 model mounted in our return loop wind tunnel

The experiments are conducted at both low angles of attack in the pre-stall regime and high angles of attack in the post-stall regime. In the former, the flow is often nominally attached with the presence of either laminar separation bubbles or mild trailing edge separation, while flow in the latter regime is fully separated.

The presence of boundary layer separation at low Reynolds number is known to affect the unsteady aerodynamic forces and moment, when compared to the fully-attached flow cases. Hence, a deeper understanding of the important flow features that affect the dynamics of the aerodynamic loads is important. Such an understanding of the flow physics will enable development of better physics-based aerodynamic models in the future that can be subsequently used for better aerodynamic, structural, and controller designs.

Aircraft Aeroacoustics

Airframe Noise

View of the anechoic wind tunnel at the University of Toronto Institute for Aerospace Studies (UTIAS).

Since the dawn of commercial air travel in the 1970s, aircraft noise emission has become a pressing environmental and health issue of the general populace – especially with the growing scale of commercial and regional airline fleets operating from airports near densely populated areas. Of the several noise sources responsible for aircraft noise, high-lift device (HLD) noise is one of the most significant – especially during approach and landing.

CAA Simulation of side-edge showing the side edge vortex and aeroacoustics field

For small to mid-size commercial aircraft, the dominant sources of HLD noise during the approach and landing phases are the flaps and flap side-edges 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 and hybrid anechoic wind tunnel, which allows for accurate measurements of aerodynamic and aeroacoustic quantities. 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.

Laminar Separation Bubbles

Interferometry — Typical oil film interferometry image showing fringe lines produced under the effect of surface shear stress on a thin layer of oil. This technique is used to measure the shear stress acting on the airfoil surface.

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.

Airfoil Model — Airfoil model mounted in the wind tunnel. The backlighting reveals the internal tubing for surface pressure measurements.

Bio-inspired Aerodynamics

Seagull Wing — Seal gull wing being tested in our closed-loop wind tunnel. Black dots were applied on the wings to track deflections under aerodynamic loading.

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.

Active Forcing of Turbulent Boundary Layers

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.

Past Projects

Nonparallel Cylinders in Tandem Arrangement Blunt Trailing Edge Wake Control The Round Turbulent Jet High Performance Sports Aerodynamics

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%.

Isosurfaces of the vorticity components in the wake of a blunt trailing edge body at Re=2500 with forcing, showing the presence of streamwise vortex loops connecting the von Karman vortices. The structure is constructed from phase-averaged planar stereo-PIV data.

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.

Plif measurements — PLIF measurements of concentration in a turbulent jet.

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.

High Performance Sports Aerodynamics

Bicycle Model — Half-scale model of the DiamondBack triathlon bicycle being tested in the our closed-loop wind tunnel.

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.

Bicycle Model inside wind tunnel — Half-scale model of the DiamondBack triathlon bicycle being installed in the closed-loop wind tunnel.