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 Title
 Effect Of Synthetic Jet Modulation Schemes On The Reduction Of A Laminar Separation Bubble.
 Creator

Seo, J. H., Cadieux, F., Mittal, R., Deem, E., Cattafesta, L.
 Abstract/Description

The response of a laminar separation bubble to synthetic jet forcing with various modulation schemes is investigated via direct numerical simulations. A simple sinusoidal waveform is considered as a reference case, and various amplitude modulation schemes, including the squarewave "burst" modulation, are employed in the simulations. The results indicate that burst modulation is less effective at reducing the length of the flow separation than the sinusoidal forcing primarily because burst...
Show moreThe response of a laminar separation bubble to synthetic jet forcing with various modulation schemes is investigated via direct numerical simulations. A simple sinusoidal waveform is considered as a reference case, and various amplitude modulation schemes, including the squarewave "burst" modulation, are employed in the simulations. The results indicate that burst modulation is less effective at reducing the length of the flow separation than the sinusoidal forcing primarily because burst modulation is associated with a broad spectrum of input frequencies that are higher than the target frequency for the flow control. It is found that such highfrequency forcing delays vortex rollup and promotes vortex pairing and merging, which have an adverse effect on reducing the separation bubble length. A commonly used amplitude modulation scheme is also found to have reduced effectiveness due to its spectral content. A new amplitude modulation scheme which is tailored to impart more energy at the target frequency is proposed and shown to be more effective than the other modulation schemes. Experimental measurements confirm that modulation schemes can be preserved through the actuator and used to enhance the energy content at the target modulation frequency. The present study therefore suggests that the effectiveness of synthetic jetbased flow control could be improved by carefully designing the spectral content of the modulation scheme.
Show less  Date Issued
 20180312
 Identifier
 FSU_libsubv1_wos_000427117100001, 10.1103/PhysRevFluids.3.033901
 Format
 Citation
 Title
 Exploration Of Thermal Counterflow In He Ii Using Particle Tracking Velocimetry.
 Creator

Mastracci, Brian, Guo, Wei
 Abstract/Description

Flow visualization using particle image velocimetry (PIV) and particularly particle tracking velocimetry (PTV) has been applied to thermal counterflow in He II for nearly two decades now, but the results remain difficult to interpret because tracer particle motion can be influenced by both the normal fluid and superfluid components of He II as well as the quantized vortex tangle. For instance, in one early experiment it was observed (using PTV) that tracer particles move at the normal fluid...
Show moreFlow visualization using particle image velocimetry (PIV) and particularly particle tracking velocimetry (PTV) has been applied to thermal counterflow in He II for nearly two decades now, but the results remain difficult to interpret because tracer particle motion can be influenced by both the normal fluid and superfluid components of He II as well as the quantized vortex tangle. For instance, in one early experiment it was observed (using PTV) that tracer particles move at the normal fluid velocity v(n), while in another it was observed (using PIV) that particles move at v(n)/2. Besides the different visualization methods, the range of applied heat flux investigated by these experiments differed by an order of magnitude. To resolve this apparent discrepancy and explore the statistics of particle motion in thermal counterflow, we apply the PTV method to a wide range of heat flux at a number of different fluid temperatures. In our analysis, we introduce a scheme for analyzing the velocity of particles presumably moving with the normal fluid separately from those presumably influenced by the quantized vortex tangle. Our results show that for lower heat flux there are two distinct peaks in the streamwise particle velocity probability density function (PDF), with one centered at the normal fluid velocity v(n) (named G2 for convenience) while the other is centered near v(n)/2 (G1). For higher heat flux there is a single peak centered near v(n)/2 (G3). Using our separation scheme, we show quantitatively that there is no size difference between the particles contributing to G1 and G2. We also show that nonclassical features of the transverse particle velocity PDF arise entirely from G1, while the corresponding PDF for G2 exhibits the classical Gaussian form. The G2 transverse velocity fluctuation, backed up by second sound attenuation in decaying counterflow, suggests that largescale turbulence in the normal fluid is absent from the twopeak region. We offer a brief discussion of the physical mechanisms that may be responsible for our observations, revealing that G1 velocity fluctuations may be linked to fluctuations of quantized vortex line velocity, and suggest a number of numerical simulations that may reveal the underlying physics in detail.
Show less  Date Issued
 20180622
 Identifier
 FSU_libsubv1_wos_000436043200001, 10.1103/PhysRevFluids.3.063304
 Format
 Citation
 Title
 Effects Of Nonuniform Viscosity On Ciliary Locomotion.
 Creator

Shoele, Kourosh, Eastham, Patrick S.
 Abstract/Description

The effect of nonuniform viscosity on the swimming velocity of a free swimmer at zero Reynolds number is examined. Using the generalized reciprocal relation for Stokes flow with nonuniform viscosity, we formulate the locomotion problem in a fluid medium with spatially varying viscosity. Assuming the limit of small variation in the viscosity of the fluid as a result of nonuniform distribution of nutrients around a swimmer, we derive a perturbation model to calculate the changes in the swimming...
Show moreThe effect of nonuniform viscosity on the swimming velocity of a free swimmer at zero Reynolds number is examined. Using the generalized reciprocal relation for Stokes flow with nonuniform viscosity, we formulate the locomotion problem in a fluid medium with spatially varying viscosity. Assuming the limit of small variation in the viscosity of the fluid as a result of nonuniform distribution of nutrients around a swimmer, we derive a perturbation model to calculate the changes in the swimming performance of a spherical swimmer as a result of positiondependent viscosity. The swimmer is chosen to be a spherical squirmer with a steady tangential motion on its surface modeling ciliary motion. The nutrient concentration around the body is described by an advectiondiffusion equation. The roles of the surface stroke pattern, the specific relationship between the nutrient and viscosity, and the Peclet number of the nutrient in the locomotion velocity of the squirmer are investigated. Our results show that for a pure treadmill stroke, the velocity change is maximum at the limit of zero Peclet number and monotonically decreases toward zero at very high Peclet number. When higher surface stroke modes are present, larger modification in swimming velocity is captured at high Peclet number where two mechanisms of thinning the nutrient boundary layer and appearance of new stagnation points along the surface of squirmer are found to be the primary reasons behind the swimming velocity modifications. It is observed that the presence of nonuniform viscosity allows for optimal swimming speed to be achieved with stroke combinations other than pure treadmill.
Show less  Date Issued
 20180424
 Identifier
 FSU_libsubv1_wos_000430691900001, 10.1103/PhysRevFluids.3.043101
 Format
 Citation
 Title
 Intermittency Enhancement In Quantum Turbulence In Superfluid He4.
 Creator

Varga, Emil, Gao, Jian, Guo, Wei, Skrbek, Ladislav
 Abstract/Description

Intermittency is a hallmark of turbulence, which exists not only in turbulent flows of classical viscous fluids but also in flows of quantum fluids such as superfluid He4. Despite the established similarity between turbulence in classical fluids and quasiclassical turbulence in superfluid He4, it has been predicted that intermittency in superfluid He4 is temperature dependent and enhanced for certain temperatures, which is in striking contrasts to the nearly flowindependent intermittency...
Show moreIntermittency is a hallmark of turbulence, which exists not only in turbulent flows of classical viscous fluids but also in flows of quantum fluids such as superfluid He4. Despite the established similarity between turbulence in classical fluids and quasiclassical turbulence in superfluid He4, it has been predicted that intermittency in superfluid He4 is temperature dependent and enhanced for certain temperatures, which is in striking contrasts to the nearly flowindependent intermittency in classical turbulence. Experimental verification of this theoretical prediction is challenging since it requires wellcontrolled generation of quantum turbulence in He4 and flow measurement tools with high spatial and temporal resolution. Here we report an experimental study of quantum turbulence generated by towing a grid through a stationary sample of superfluid He4. The decaying turbulent quantum flow is probed by combining a recently developed He*(2) molecular tracerline tagging velocimetry technique and a traditional secondsound attenuation method. We observe quasiclassical decays of turbulent kinetic energy in the normal fluid and of vortex line density in the superfluid component. For several time instants during the decay, we calculate the transverse velocity structure functions. Their scaling exponents, deduced using the extended selfsimilarity hypothesis, display nonmonotonic temperaturedependent intermittency enhancement, in excellent agreement with a recent theoretical and numerical study
Show less  Date Issued
 20180904
 Identifier
 FSU_libsubv1_wos_000443685600007, 10.1103/PhysRevFluids.3.094601
 Format
 Citation