Peter Oshkai

Shallow cavity flow tone experiments: onset of locked-on states

Abstract Fully turbulent inflow past a shallow cavity is investigated for the configuration of an axisymmetric cavity mounted in a pipe. Emphasis is on conditions giving rise to coherent oscillations, which can lead to locked-on states of flow tones in the pipe–cavity system. Unsteady surface pressure measurements are interpreted using three-dimensional representations of amplitude–frequency, and velocity; these representations are constructed for a range of cavity depth. Assessment of these data involves a variety of approaches. Evaluation of pressure gradients on plan views of the three-dimensional representations allows extraction of the frequencies of the instability (Strouhal) modes of the cavity oscillation. These frequency components are correlated with traditional models originally formulated for cavities in a free-stream. In addition, they are normalized using two length scales: inflow boundary-layer thickness and pipe diameter. These scales are consistent with those employed for the hydrodynamic instability of the separated shear layer, and are linked to the large-scale mode of the shear layer oscillation, which occurs at relatively long cavity length. In fact, a simple scaling based on pipe diameter can correlate the frequencies of the dominant peaks over a range of cavity depth. The foregoing considerations provide evidence that pronounced flow tones can be generated from a fully turbulent inflow at very low Mach number, including the limiting case of fully developed turbulent flow in a pipe. These tones can arise even for the extreme case of a cavity having a length over an order of magnitude longer than its depth. Suppression of tones is generally achieved if the cavity is sufficiently shallow.

Flow Structures in a U-Shaped Fuel Cell Flow Channel: Quantitative Visualization Using Particle Image Velocimetry

Flow through an experimental model of a U-shaped fuel cell channel is used to investigate the fluid dynamic phenomena that occur within serpentine reactant transport channels of fuel cells. Achieving effective mixing within these channels can significantly improve the performance of the fuel cell and proper understanding and characterization of the underlying fluid dynamics is required. Classes of vortex formation within a U-shaped channel of square cross section are characterized using high-image-density particle image velocimetry. A range of Reynolds numbers, 109 Re 872, corresponding to flow rates encountered in a fuel cell operating at low to medium current densities is investigated. The flow fields corresponding to two perpendicular cross sections of the channel are characterized in terms of the instantaneous and time-averaged representations of the velocity, streamline topology, and vorticity contours. The critical Reynolds number necessary for the onset of instability is determined, and the two perpendicular flow planes are compared in terms of absolute and averaged velocity values as well as Reynolds stress correlations. Generally, the flow undergoes a transition to a different regime when two recirculation zones, which originally develop in the U-bend region, merge into one separation region. This transition corresponds to generation of additional vortices in the secondary flow plane. fDOI: 10.1115/1.1843121g

Optical distortion correction for liquid droplet visualization using the ray tracing method: further considerations

The original work of Kang et al (2004 Meas. Sci. Technol. 15 1104–12) presents a scheme for correcting optical distortion caused by the curved surface of a droplet, and illustrates its application in PIV measurements of the velocity field inside evaporating liquid droplets. In this work we re-derive the correction algorithm and show that several terms in the original algorithm proposed by Kang et al are erroneous. This was not evident in the original work because the erroneous terms are negligible for droplets with approximately hemispherical shapes. However, for the more general situation of droplets that have shapes closer to that of a sphere, with heights much larger than their contact-line radii, these errors become quite significant. The corrected algorithm is presented and its application illustrated in comparison with that of Kang et al.

Flow within a water droplet subjected to an air stream in a hydrophobic microchannel

Two-phase air–water flow in an experimental model of a polymer electrolyte membrane fuel cell (PEMFC) gas distribution channel is investigated using quantitative flow imaging of the liquid phase. A rectangular gas channel model was fabricated from polydimethylsiloxane (PDMS), glass and carbon paper. A micro-digital-particle-image-velocimetry (micro-DPIV) technique was used to provide qualitative and quantitative visualizations of flow inside a water droplet adhering to the bottom wall of a gas channel and exposed to an air flow within the channel. Velocity measurements in a central cross-sectional plane inside a droplet placed in the channel are reported for a range of air flow rates. The relationships between air velocity in the channel, secondary rotational flow inside a droplet, droplet deformation and contact angle hysteresis are examined. The resulting flow fields provide insight into the interactions between the air and water flows that occur at the gas–liquid interface.

Flow tones in a pipeline-cavity system: effect of pipe asymmetry

Abstract Flow tones in a pipeline-cavity system are characterized in terms of unsteady pressure within the cavity and along the pipe. The reference case corresponds to equal lengths of pipe connected to the inlet and outlet ends of the cavity. Varying degrees of asymmetry of this pipe arrangement are investigated. The asymmetry is achieved by an extension of variable length, which is added to the pipe at the cavity outlet. An extension length as small as a few percent of the acoustic wavelength of the resonant mode can yield a substantial reduction in the pressure amplitude of the flow tone. This amplitude decrease occurs in a similar fashion within both the cavity and the pipe resonator, which indicates that it is a global phenomenon. Furthermore, the decrease of pressure amplitude is closely correlated with a decrease of the Q (quality)-factor of the predominant spectral component of pressure. At a sufficiently large value of extension length, however, the overall form of the pressure spectrum recovers to the form that exists at zero length of the extension. Further insight is provided by variation of the inflow velocity at selected values of extension length. Irrespective of its value, both the magnitude and frequency of the peak pressure exhibit a sequence of resonant-like states. Moreover, the maximum attainable magnitude of the peak pressure decreases with increasing extension length.

The influence of the aortic root geometry on flow characteristics of a prosthetic heart valve.

In this paper, performance of aortic heart valve prosthesis in different geometries of the aortic root is investigated experimentally. The objective of this investigation is to establish a set of parameters, which are associated with abnormal flow patterns due to the flow through a prosthetic heart valve implanted in the patients that had certain types of valve diseases prior to the valve replacement. Specific valve diseases were classified into two clinical categories and were correlated with the corresponding changes in aortic root geometry while keeping the aortic base diameter fixed. These categories correspond to aortic valve stenosis and aortic valve insufficiency. The control case that corresponds to the aortic root of a patient without valve disease was used as a reference. Experiments were performed at test conditions corresponding to 70 beats/min, 5.5 L/min target cardiac output, and a mean aortic pressure of 100 mmHg. By varying the aortic root geometry, while keeping the diameter of the orifice constant, it was possible to investigate corresponding changes in the levels of Reynolds shear stress and establish the possibility of platelet activation and, as a result of that, the formation of blood clots.

Investigation of Diametral Acoustic Modes in a Model of a Steam Control Gate Valve

The objective of the present study is to provide an insight into mechanism of coupling between turbulent pipe flow and partially trapped diametral acoustic modes associated with a shallow cavity formed by the seat of a steam control gate valve. First, the effects of the internal pipe geometry immediately upstream and downstream of the shallow cavity on the characteristics of partially trapped diametral acoustic modes were investigated. The mode shapes were calculated numerically by solving a Helmholtz equation in a three-dimensional domain corresponding to the internal geometry of the pipe and the cavity. Second, the set of experiments were performed using a scaled model of a gate valve mounted in a pipeline that contained converging–diverging sections in the vicinity of the valve. Acoustic pressure measurements at three azimuthal locations at the floor of the cavity were performed for a range of geometries of the converging–diverging section and inflow velocities. The experimentally obtained pressure data were then used to scale the amplitude of the pressure in the numerical simulations. The present results are in good agreement with the results reported in earlier studies for an axisymmetric cavity mounted in a pipe with a uniform cross-section. The resonant response of the system corresponded to the second diametral mode of the cavity. Excitation of the dominant acoustic mode was accompanied by pressure oscillations corresponding to other acoustic modes. As the angle of the converging–diverging section of the main pipeline in the vicinity of the cavity increased, the trapped behavior of the acoustic diametral modes diminished, and additional antinodes of the acoustic pressure wave were observed in the main pipeline.

Effect of transverse perforations on fluid loading on a long, slender plate at zero incidence

Abstract This paper reports the results of experimental investigations of flow-induced loading on perforated and solid flat plates at zero incidence with respect to the incoming flow. The plates had a streamwise length to transverse thickness ratio of 23.5. The effect of the perforations was investigated for three different perforation diameters. The results corresponding to the perforated plates were compared with the reference case of the solid plate (no perforations) at five inflow velocities. We quantified the effect of the perforations on the unsteady fluid loading on the plate in terms of the variations of the corresponding Strouhal number, the mean drag coefficient and the fluctuating lift coefficient as functions of the Reynolds number and the perforation diameter. The results indicate that the loading was dominated by the dynamics of the wake. In particular, increasing the perforation diameter resulted in a wider wake, corresponding to the increase in mean drag coefficient and the decrease in the Strouhal number. Onset of coupling between the vortex shedding and the transverse oscillations of the plate was manifested as a rapid increase in the fluctuating lift coefficient, as the perforation diameter exceeds the plate thickness.

Design and application of nanoparticle coating system with decoupled spray generation and deposition control

Coatings are widely used in various biomedical applications to change the interaction of the surfaces with bioactive materials. The key factors that determine the quality of a spray-coated layer are the size (order of a few microns in diameter) and dimensional uniformity of droplets in the spray and the droplet impact velocity. For many applications, coating quality is strongly dependent on the method and equipment used during the application process. This paper presents the development of a decoupled system for spray coating and micro-printing, which includes an ultrasonic spray generation device and a nozzle for the spray deposition independently operated. Design and development of the system as well as testing for different applications are presented in this paper. The system design can be potentially used for large area coating, such as windows and solar panels, as well as micro-printing of electronic circuits and numerous other applications.

Experimental Investigation of Flow-Acoustic Coupling in a Deep Axisymmetric Cavity

In this paper, the phenomenon of self-sustained pressure oscillations due to the flow past a deep, circular, axisymmetric cavity is investigated. In many engineering applications, such as flows through open gate valves, there exists potential for coupling between the vortex shedding from the upstream edge of the cavity and a diametral mode of the acoustic pressure fluctuations. In the present study, the unsteady pressure was measured at several azimuthal locations at the bottom of the cavity walls, and the associated acoustic mode shapes were calculated numerically for the four representative cases of the internal cavity geometry, which involved a reference case with sharp, 90°edges as well as several modifications that involved chamfers of various length of the upstream and the downstream edges of the cavity. In addition, the flow velocity in the vicinity of the cavity opening in selected cases was measured using digital particle image velocimetry (PIV). The optical access to the highly confined internal flow was provided by implementing an endoscope attached to the camera. This global, quantitative imaging approach yielded patterns of velocity, streamlines and out-of-plane vorticity component. Instantaneous and time-averaged flow patterns provided insight into the mechanism of the flow tone generation. Among the considered cavity geometries, the configuration that corresponded to the most efficient noise suppression was identified.Copyright