Fluid Dynamics Researchers Meet in San Francisco
New research results in turbulence control, sonoluminescence, and biofluid dynamics were among the highlights of the 1997 fall meeting of the APS Division of Fluid Dynamics, held 23-25 November in San Francisco. Nearly 900 contributed papers were presented in addition to several invited lectures. In addition, the 1997 recipients of the APS Fluid Dynamics Prize and Otto LaPorte Award spoke at a special awards program on Sunday afternoon. The meeting also featured the 15th Annual Gallery of Fluid Motion, an exhibit of contributed photographs and videos of experimental fluid dynamics. Outstanding entries, selected for originality and their ability to convey and exchange information, will appear in the September 1998 issue of Physics of Fluids.
MEMs and Turbulence Control
Recent experiments and simulations have demonstrated the feasibility of active boundary layer control, according to Sudeep Kumar and William Reynolds of Stanford University, who spoke at a Tuesday afternoon session. They have developed actuator arrays using a combination of micromachining technologies along with mesoscale assembly. An array consists of eight piezoceramic-silicon cantilevers with integrated cavities and unequal side gaps, with typical spring constants ranging between 100-500 N/m. In addition, the actuator has millisecond rise times with power consumption in the milliwatt range.
At the same session, Steve Tung of CalTech described how his team has designed and fabricated multiple arrays of micromachined micro shear-stress sensors, intended to temporally and spatially resolve the small stream-wise streaks in the near-wall region of a turbulent boundary layer. Using these sensors, the turbulent surface shear-stress distribution has been measured, and the high shear-stress streaks have been identified and analyzed. Based on the temporal data, Tung's group found that a high correlation exists between the peak shear stress level and the leading edge shear-stress gradient of a high shear-stress streak. This information is currently being applied to the design of a real-time flow control logic, which is part of a MEMS-based neuronet system for active turbulent shear-stress control.
Synthetic Jet Actuators
Ari Glezer of the Georgia Institute of Technology described his novel approach to the manipulation and control of shear flows using fluidic technology based on synthetic jets. These jets are zero-mass-flux and are synthesized from the working fluid in the flow system in which they are embedded. Although there is no net mass injection, the jets provide for momentum transfer into the flow system to be controlled. In addition, near the surface from which it is generated, interaction of a synthetic jet with an embedding flow results in formation of closed recirculation regimes, with an apparent modification of the surface shape.
These attributes enable synthetic-jet control systems to effect significant global modification of embedding flows on scales one to two orders of magnitude larger than the characteristic length scale of the jets. Futhermore, while conventional excitation methods have been limited to frequency bands tailored to the linear receptivity mechanisms of a given flow, fluidic actuation facilitates exploitation of nonlinear mechanisms for amplification of disturbances in a very broad frequency band. Potential applications of fluidic technology based on synthetic jets include jet mixing and thrust management, and modification of aerodynamic surfaces.
When an acoustic wave of moderate pressure converges in an aqueous liquid, light emissions can be observed, a conversion of mechanical energy into electromagnetic energy that represents an energy amplification per molecule of over eleven orders of magnitude. On Monday afternoon, Lawrence Crum of the University of Washington's Applied Physics Laboratory, described his recent discovery that a single, stable gas bubble, acoustically levitated in a liquid, can emit optical emissions each cycle for an unlimited period of time. "We have no current explanation for how this mechanical system sustains itself," Crum admitted. "Presumably, the oscillations of the bubble cause the gas in the interior to be heated to incandescent temperatures during the compression portion of the cycle."
Furthermore, recent experimental evidence suggests that the lifetime of the optical pulse is less than 12 picoseconds, and that the temperature in the interior of the bubble can exceed 40,000 K. While Crum finds the recent suggestion that sonoluminescence may be due to quantum vacuum radiation, he theorizes that a shock wave is created in the gas, which is then elevated to high temperatures by inertial confinement. "If shock waves are the mechanism for sonoluminescent emission, the optimization of the process could lead to extraordinary physics, including the remote but intriguing possibility of thermonuclear fusion," he said.
According to Charles Peskin of New York University, who spoke on Monday afternoon, the fluid dynamics of the heart involve the interaction of blood, a viscous incompressible fluid, with the flexible, elastic, fiber- reinforced heart valve leaflets that are immersed in that fluid. Neither the fluid motion nor the valve leaflet motion are known in advance; both must be computed simultaneously by solving their coupled equations of motion. Peskin has developed a means of accomplishing this simulation using his immersed boundary method, which can be extended to incorporate the contractile fiber architecture of the muscular heart walls, as well as the valve leaflets and the blood. The result is a three-dimensional model of the heart, which can be used as a test chamber for the design of prosthetic cardiac valves, and also to study the function of the heart in health and in disease.
Dogs and other scenting animals detect airborne odors with extraordinary sensitivity. According to G.W. Settles of Penn State University, who spoke on Tuesday afternoon, aerodynamic sampling plays a key role in this, although little information is available on the external aerodynamics thereof. To this end, he visualized the airflows generated by a scenting dog using the so-called "schlieren technique." He observed that a dog stops panting in order to scent, since panting produces a turbulent jet which disturbs scent-bearing air currents. Furthermore, inspiratory airflow enters the nostrils from straight ahead, while expiration is directed to the sides of the nose and downward. Thus, the musculature and geometry of a dog's nose modulates the airflow during scenting. The eventual practical application of his work is to achieve a sufficient level of understanding of the aerodynamics of canine olfaction to design a mimicking device.
On Tuesday afternoon, William Saric of Arizona State University described recent research in the three-dimensional boundary layer transition, focusing on the cross-flow instability that leads to nonlinear saturation and transition on swept wings with pressure gradients. In particular, he has shown that the introduction of micron sized roughness organizes the unstable modes in up to nine harmonics, and that it is possible to isolate single-mode growth in order to provide a data base for the computations. According to Saric, the measurements show a clear nonlinear distortion of the mean flow and saturation of the stationary structure. He has also shown that certain roughness spacings inhibit the growth of the most unstable modes, and that transition can be moved beyond the smooth case.