Research@FPL
Fluid Structure Interaction
Several problems of engineering interest have strong interactions between the fluid flow around surfaces/bodies, and the motions/deformations of the solid surfaces/bodies. Examples of such problems range from compressor blade flutter of aircraft engines to the gentle flapping of leaves in the wind. In these problems, the strong interactions between the fluid and the solid surfaces can result in rich and interesting dynamics both within the fluid and in the deformations or motions of the solid surfaces. We are exploring a range of such Fluid-Structure Interaction problems in the lab. These include the flapping of flexible foils for the generation of efficient thrust in “robotic fishes”, the unsteady shock induced flutter of compressor blades, stall flutter of single blades or airfoils and the flow through flexible valves. One of the focus areas in FSI within the lab has been on studying the effects of surface flexibility in a few flow configurations motivated by biological systems, whose surfaces (tissues/membranes) are inherently flexible. In this connection, we have studied
the effects of vortex-pair/vortex-ring generation from a flexible exit, motivated by the propulsive action of some aquatic creatures such as the jelly-fish, and heart valves that are composed of flexible membranes.
the forces and wake of an oscillating flexible airfoil motivated by the undulating motions of a “flexible” fish tail to generates propulsive thrust.
the effect of a flexible splitter plate in the wake of cylinder for reducing drag and to suppress noise and vibrations.
Bubbly Turbulent Flow
There has been a lot of recent interest in reducing drag on surfaces using bubbles with potential applications ranging from drag reduction of underwater vehicles to pressure drop reduction in liquid pipelines. This includes primarily two broad methods, one utilizing “textured hydrophobic” surfaces, and the other more classical approach is to inject small bubbles from the surface into the flow. We have done work related to both approaches as detailed below.
- Bubble injection into the flow
Single bubble – Vortex ring studies: An idealization of the interaction of bubbles with turbulent structures is the interaction of a single bubble with a single vortical structure, namely a vortex ring (formed in water). In these studies, measurements of both the bubble dynamics and vorticity dynamics have been done to help understand the two-way coupled problem over a large range of vortex ring and bubble parameters. The results from these studies show that they exhibit many phenomena also seen in bubbly turbulent flows such as reduction in enstrophy, suppression of structures, enhancement of energy at small scales and reduction in energy at large scales. These similarities suggest that results from the present experiments can be helpful in better understanding interactions of bubbles with eddies in turbulent flows.
Drag reduction using bubbles in turbulent channel: We have also studied actual drag reduction in a turbulent channel flow by the injection of bubbles. We have systematically mapped out the effect of different rates of bubble injection, on the drag reduction (pressure drop) achieved within a turbulent channel. These results show that there exist many different regimes of bubble dynamics in the flow depending on the rate of bubble injection and the channel Reynolds numbers. The measured drag (or pressure drop) is found to be greatly dependent on the bubble dynamics regime with possibilities of both enhanced drag and reduced drag compared to the reference no bubble case. When the conditions are right, very large drag reductions of up to 60 % were achieved.
- Trapped bubbles on the surface – Superhydrophobic surfaces:
In the approach using injected bubbles into the flow, drag reduction by bubbles can either be caused by direct modification of fluid properties like density and viscosity, or through the relatively more complex interaction of bubbles with turbulent structures within the boundary layer. In this broad area, we have studied:
In contrast to the injection of bubbles, another mechanism for drag reduction is the use of “textured hydrophobic surfaces” as channel walls. The ability of these surfaces to provide substantial drag reduction has been attributed to the presence of air bubbles trapped on the surface cavities. However, this drag reduction cannot be sustained due to gradual dissolution of trapped air into water. In our recent work, we have explored the possibility of sustaining the underwater Cassie state of wetting in a microchannel by controlling the solubility of air in water; the solubility being changed by controlling the local absolute pressure near the surface. We show that using this method, we can in fact make the water locally supersaturated with air thus encouraging the growth of trapped air pockets on the surface. In this case, the water acts as a pumping medium, delivering air to the crevices of the hydrophobic surface in the microchannel, where the presence of air pockets is most beneficial from the drag reduction perspective.
High speed flows & Turbomachinery
We have been studying high-speed (transonic/supersonic) flows with shocks focusing on the unsteady aspects of these flows. Some of the main projects in this area are:
- Shock Boundary layer interactions over a forward-facing step
- Transonic Flutter of aero-engine compressor blades
- Jet in supersonic cross-flow
- Stall in high-speed centrifugal compressors
- Shock Boundary layer interactions over a forward-facing step
- Transonic Flutter of aero-engine compressor blades
- Jet in supersonic cross-flow
We have studied a Mach 2.5 flow over a forward-facing step (Narayan & Govardhan, 2015). The focus of the work was the flow ahead of the step, in particular, the interactions between the shock, the boundary layer, and the three-dimensional separation bubble with time. Detailed PIV measurements in two orthogonal planes were done in addition to unsteady wall pressure measurements upstream of the step. The mean velocity field in the central x-y plane shows that the incoming boundary layer separates upstream of the step forming a large separation bubble ahead of the step, which can be relatively well resolved in PIV measurements compared to the compression ramp cases. On the temporal side, the wall pressure fluctuation spectra close to the separation location show a dominant frequency that is two orders smaller than the characteristic frequency of the incoming boundary layer (fδ/U ~ 0.01), consistent with low frequency motions of the shock that has received a lot of recent attention (Clemens & Narayanaswamy 2014). PIV measurements in the cross-stream plane show large variations in shock position with time, the shock being measured well outside the boundary layer. This shock is found to be well correlated with the reversed flow area ahead of the step, as shown by conditionally-averaged PIV fields. Measurements in the other plane parallel to the lower wall show that the shock is broadly two-dimensional with small spanwise ripples, while the recirculation region is highly three-dimensional. The spanwise-averaged shock location is found to be determined mainly by the most upstream location of the recirculation region over a spanwise area. Hence, the present results suggest that for the forward-facing step configuration, it is the most upstream point of the recirculation region that is very important in deciding the shock location ahead of the step. On the other hand, the shock foot is found to have ripples that are well correlated to the streaks in the incoming boundary layer.
We have experimentally studied heave mode flutter of a blade within a linear cascade at transonic conditions. Driven by the motivation to understand the contribution of shock location/dynamics to flutter characteristics, we have performed simultaneous measurements of shock dynamics using high-speed shadowgraphy combined with unsteady load measurements on an oscillating blade within the cascade. The flutter characteristics in terms of energy transfer from the fluid to the blade and shock dynamics have been mapped out over a range of blade oscillation frequencies and static pressure ratios (SPR) across the cascade, the latter being important as it decides the mean location of the passage shocks. SPR values studied include both conditions where the shock is within the passage (started cascade) and where the passage shock is pushed ahead of the leading edge of the blades (unstarted cascade). These measurements show characteristically different flutter behavior for an unstarted cascade compared with a started cascade, the former having received very little attention in the literature. While both these cases show small excitation levels at low reduced frequencies, the unstarted cascade case exhibits an additional relatively narrow region of excitation at higher reduced frequencies with approximately an order of magnitude higher excitation energies. Comparison of the shock dynamics between the two excitation regimes show significant differences in the phase of the leading edge shock in addition to changes in the suction side shock phase indicating that the two excitation regimes are of different origin.
Supersonic air-breathing engines or scramjets are perhaps the most important technological hurdles towards the development of hypersonic transportation vehicles. In these engines, the air entering the combustor must remain supersonic, which significantly brings down the time available within the engine for fuel-air mixing and combustion. There are many possible strategies for fuel injection in such engines; the transverse jet injection into supersonic crossflow being one. We have experimentally studied the flowfield and mixing associated with such a sonic jet injection into a supersonic crossflow. In addition to detailed flowfield and mixing studies of the basic steady jet configuration, enhancement of mixing using a newly developed passively modulated (injection) jet has also been studied. In all cases, the flowfield has been investigated using Particle Image Velocimetry (PIV) to characterize the different flow features and the penetration of the jet into the crossflow, while mixing studies have also been carried out using acetone Planar Laser Induced Fluorescence (PLIF).
