Tyler Van Buren

Turbulent drag reduction over air- and liquid- impregnated surfaces

Results on turbulent skin friction reduction over air- and liquid-impregnated surfaces are presented for aqueous Taylor-Couette flow. The surfaces are fabricated by mechanically texturing the inner cylinder and chemically modifying the features to make them either non-wetting with respect to water (air-infused, or superhydrophobic case), or wetting with respect to an oil that is immiscible with water (liquid-infused case). The drag reduction, which remains fairly constant over the Reynolds number range tested (100 ≤ Reτ ≤ 140), is approximately 10% for the superhydrophobic surface and 14% for the best liquid-infused surface. Our results suggest that liquid-infused surfaces may enable robust drag reduction in high Reynolds number turbulent flows without the shortcomings associated with conventional superhydrophobic surfaces, namely, failure under conditions of high hydrodynamic pressure and turbulent flow fluctuations.

Vortex formation of a finite-span synthetic jet: High Reynolds numbers

The formation and evolution of flow structures of a high-speed, finite-span synthetic jet issued into a quiescent flow were investigated experimentally using Stereoscopic Particle Image Velocimetry for jet peak velocities up to 150 m/s. The effect of high jet Reynolds number (Re○ = 1150, 3450, and 5750, with corresponding Strouhal numbers of St = 0.215, 0.046, and 0.028, respectively) on a finite-span synthetic jet with aspect ratios of 18 and 24 were explored. It was found that the velocity and vorticity fields were greatly affected by the Reynolds number, with the lowest Reynolds number case producing the highest normalized peak velocities and vorticity. Moreover, for all three Reynolds numbers, axis switching was observed and its streamwise location increased as the Reynolds number increased. The Q-criterion was utilized for decoupling vortices from the vorticity concentrations in the flow field. This enabled the identification of vortices in the flow field and the reconstruction of the 3D vortex ring ...

Scaling the propulsive performance of heaving and pitching foils

Scaling laws for the propulsive performance of rigid foils undergoing oscillatory heaving and pitching motions are presented. Water tunnel experiments on a nominally two-dimensional flow validate the scaling laws, with the scaled data for thrust, power, and efficiency all showing excellent collapse. The analysis indicates that the behaviour of the foils depends on both Strouhal number and reduced frequency, but for motions where the viscous drag is small the thrust closely follows a linear dependence on reduced frequency. The scaling laws are also shown to be consistent with biological data on swimming aquatic animals.

Three-dimensional interaction of a finite-span synthetic jet in a crossflow

The formation and evolution of flow structures due to the interaction of a finite-span synthetic jet with a zero-pressure gradient laminar boundary layer were experimentally investigated using stereoscopic particle image velocimetry. A synthetic jet with three orifice aspect ratios of AR = 6, 12, and 18 was issued into a free-stream velocity of U∞ = 10 m/s (Reδ = 2000) at blowing ratios of Cb = 0.5–1.5. The interaction was found to be associated with two sets of flow structures: (1) a recirculation region downstream of the orifice due to virtual blockage, and (2) a steady streamwise vortex pair farther downstream. These two flow structures were characterized in detail. Tube-like velocity deficits in the free-stream were evident, as well as regions of increased velocity within the boundary layer. Reducing the aspect ratio of the orifice decreased the spacing of the edgewise vortices (generated due to the finite span of the orifice) as well as reducing the virtual blockage of the jet. A control volume analysis of the fluid streamwise momentum indicates that there is a momentum deficit just downstream of the jet orifice and the change in streamwise momentum is proportionally similar for all cases.

Impact of orifice orientation on a finite-span synthetic jet interaction with a crossflow

The formation and evolution of flow structures associated with a finite-span synthetic jet issued into a zero-pressure gradient boundary layer were investigated experimentally using stereoscopic particle image velocimetry. A synthetic jet with an aspect ratio of AR = 18 was mounted on a flat plate and its interaction with a free stream, having a velocity of U∞ = 10 m/s (Reδ = 2000) at momentum coefficients of Cμ = 0.08, 0.33, and 0.75, was studied. The effect of the orifice pitch (α = 20∘–90∘) and skew (β = 0∘–90∘) angles on vortex formation as well as the global impact of the synthetic jet on the flow field was explored in detail. It was found that the orifice orientation had a significant impact on the steady and unsteady flow structures. Different orifice skew and pitch angles could result in several types of vortical structures downstream, including: no coherent vortex structure, a single (positive or negative) strong vortex, or a symmetric vortex pair. In all cases, the velocity near the wall was increased; however, cases with higher blockage (i.e., more wall-normal, transverse orifice) resulted in a strong velocity deficit in the free stream where orifices with lower pitch angles yielded in an increase in velocity throughout. The analysis is concluded with a summary of quantitative metrics that allude to flow control effectiveness.

Vortex Formation of a Finite Span Synthetic Jet

The effects of different geometries and input parameters on the flow structures of a finite span synthetic jet are explored in quiescent flow experiments using stereoscopic PIV. Common geometrical parameters, such as neck height, and orifice aspect ratio are varied along with the jet performance characteristics such as Strouhal numbers and Reynolds numbers. Orifice neck heights of 2, 4 and 6mm were tested at aspect ratios of 6, 12, and 18. Jet Strouhal numbers of 0.115 to 0.157 were tested at Reynolds numbers of 615. It is found that as the aspect ratio was increased the size of the vortical structures decreases, the vorticity dissipation rate increases, and axis switching occurred farther away from the orifice. The neck length had a large effect on the structures’ size and strength, their dissipation rate, and the jet’s spreading. Moreover, the driving frequency was found to affect the vortical structures spreading due to the streamwise spacing between structures, as well as drastically affecting dissipation of the structures in both planes.

Forces and energetics of intermittent swimming

Experiments are reported on intermittent swimming motions. Water tunnel experiments on a nominally two-dimensional pitching foil show that the mean thrust and power scale linearly with the duty cycle, from a value of 0.2 all the way up to continuous motions, indicating that individual bursts of activity in intermittent motions are independent of each other. This conclusion is corroborated by particle image velocimetry (PIV) flow visualizations, which show that the main vortical structures in the wake do not change with duty cycle. The experimental data also demonstrate that intermittent motions are generally energetically advantageous over continuous motions. When metabolic energy losses are taken into account, this conclusion is maintained for metabolic power fractions less than 1.

Achieving a High-Speed and Momentum Synthetic Jet Actuator

A detailed analysis of a finite-span synthetic jet in a quiescent fluid is presented, with the goal of achieving a high speed and momentum synthetic jet, with a peak velocity exceeding 200 m/s. A total of two scales of actuator apparatuses having either 40- or 80-mm-diameter piezoelectric discs were used. A temperature-compensated hot wire, laser displacement sensor, and dynamic pressure transducers were used to quantify the performance of a given actuator. The synthetic jet generation process was divided into its main components, diaphragm displacement, cavity pressure, and orifice velocity, which were analyzed in detail in both their peak values and time responses. The phase shift between the disc displacement and velocity signals were found to be mainly attributed to compressibility effects. Doubling the diameter and thickness of the piezoelectric disc was found to double the output jet velocity and shift the operation frequency range of the system. In addition, a dual disc configuration was used, which yielded an approximately 40% higher jet velocity than its single disc configuration. Using the knowledge acquired from these experiments resulted in a synthetic jet having a peak velocity of 211 m/s out of a 12×1-mm rectangular orifice at 700 Hz, which is significantly stronger than any similar actuator reported in the literature.

Interaction between a vortex generator and a synthetic jet in a crossflow

The interaction of a combined vortex generator and a finite-span synthetic jet, i.e., a hybrid actuator, with a zero pressure gradient laminar boundary layer over a flat plate was explored experimentally using Stereoscopic Particle Image Velocimetry (SPIV). The free stream velocity was U∞ = 10 m/s corresponding to a Reynolds number based on the local boundary layer thickness Reδ ≈ 2000. The synthetic jet was activated at multiple blowing ratios, and the vortex generator (placed either upstream or downstream of the synthetic jet) had a height of 1.6 times the local boundary layer thickness. When exposed to the crossflow, the pitched and skewed synthetic jet and vortex generator independently produced a single streamwise vortex in the far field. However, when the combined synthetic jet and vortex generator were placed together on the flat plate, the two streamwise vortices, associated with the two devices, did not combine. When the vortex generator was upstream of the synthetic jet, the jet pushed the vorte...

Efficient cruising for swimming and flying animals is dictated by fluid drag

Significance Almost 30 y ago, researchers discovered that a great variety of efficient swimmers cruise in a narrow range of Strouhal numbers, a dimensionless number describing the kinematics of swimming. Almost 15 y later, separate researchers discovered that fliers (bats, birds, and insects) also cruise in the same narrow range of Strouhal numbers. Attendant experiments on flapping airfoils have shown that this narrow range of Strouhal numbers gives rise to the most efficient kinematics. Here, we explain why this range of Strouhal numbers is the most efficient. Many swimming and flying animals are observed to cruise in a narrow range of Strouhal numbers, where the Strouhal number St=2fA/U is a dimensionless parameter that relates stroke frequency f, amplitude A, and forward speed U. Dolphins, sharks, bony fish, birds, bats, and insects typically cruise in the range 0.2<St<0.4, which coincides with the Strouhal number range for maximum efficiency as found by experiments on heaving and pitching airfoils. It has therefore been postulated that natural selection has tuned animals to use this range of Strouhal numbers because it confers high efficiency, but the reason why this is so is still unclear. Here, by using simple scaling arguments, we argue that the Strouhal number for peak efficiency is largely determined by fluid drag on the fins and wings.