Yogen Utturkar

Formation Criterion for Synthetic Jets

Af ormation criterion for synthetic jets is proposed and validated. A synthetic jet actuator is a zero-net mass-flux device that imparts momentum to its surroundings. Je tf ormation is defined as the appearance of a time-averaged outward velocity along the jet axis and corresponds to the generation and subsequent convection or escape of a vortex ring. It is shown that over a wide range of operating conditions synthetic jet formation is governed by the jet Strouhal number Sr (or Reynolds number Re and Stokes number S). Both numerical simulations and experiments are performed to supplement available two-dimensional and axisymmetric synthetic jet formation data in the literature. The data support the jet formation criterion 1/Sr = Re/S 2 > K, where the constant K is approximately 1 and 0.16 for two-dimensional and axisymmetric synthetic jets, respectively. In addition, the dependence of the constant K on the normalized radius of curvature of a rounded orifice or slot is addressed. The criterion is expected to serve as a useful design guide for synthetic jet formation in flow control, heat transfer, and acoustic liner applications, in which a stronger jet is synonymous with increased momentum transfer, vorticity generation, and acoustic nonlinearities.

A Jet Formation Criterion for Synthetic Jet Actuators

This paper proposes and validates a jet formation criterion for synthetic jet actuators. The synthetic jet is a zero net mass flux device, adding additional momentum but no mass to its surroundings. Jet formation is defined as a mean outward velocity along the jet axis and corresponds to the clear formation of shed vortices. It is shown that the synthetic jet formation is governed by the Strouhal number (or Reynolds number and Stokes number). Numerical simulations and experiments are performed to supplement available two-dimensional and axisymmetric jet formation data in the literature. The data support the jet formation criterion , where the constant 2 Re/ S K > K is approximately 2 and 0.16 for two dimensional and axisymmetric synthetic jets, respectively. This criterion is valid for relatively thick orifice plates with thickness-to-width ratios greater than approximately 2. This result is expected to be useful for the design of flow-control actuators and engine nacelle acoustic liners.

Sensitivity of Synthetic Jets to the Design of the Jet Cavity

The sensitivity of synthetic jets to the design of the jet cavity is examined using numerical simulations. In this study, the primary focus is on examining the effect of changes in the cavity aspect ratio and the placement of piezoelectric diaphragm on the flow produced by the jet. Cases with and without an external cross-flow are investigated. This study compares the vortex dynamics, velocity profiles and other dynamical characteristics of the jet for the various cases and this allows us to extract some insight into the effect of these modifications on the jet performance. It is expected that this study will prove useful in the design as well as in developing dynamical models for these devices.

Impact of Turbulence and Compressibility Modeling on Three-Dimensional Cavitating Flow Computations

The large density ratio, up to 1000 in water, between liquid and vapor, turbulence with complicated interface dynamics and fast and multiple time scales make the computation of cavitating flows difficult. Utilizing a pressure-based algorithm extended for such flows, the present study focuses on the non-equilibrium and nonstationary aspects in the turbulence model, and the compressibility effects in the cavitation model. Assessment of several modeling concepts has been made in the context of the Favre-averaged Navier-Stokes equations, along with a transport equation-based cavitation model and the e − k two-equation turbulence model. The three-dimensional turbulent cavitating flow in a hollow-jet valve is adopted as the physical focus. While the non-equilibrium and nonstationary modification to the turbulence closures do not seem to influence the qualitative characteristics of the simulation, the compressibility modeling can cause the result to vary substantially.

Coupled Acoustic and Heat Transfer Modeling of a Synthetic Jet

Synthetic jets based on acoustic resonators have recently been applied to the active cooling of small, heat generating components such as microelectronics. The synthetic jets considered in this study are constructed using two piezoelectric bimorph disks separated by a donut shaped elastomeric wall with a small orifice in the wall. The piezo disks are energized to actuate out of phase at high frequency to alternately entrain and expel surrounding air. The resulting pulsating jet of air enhances local heat transfer by a factor of at least 3X, and exceeds 8x for small surfaces, compared to natural convection. These synthetic jets also create substantial noise at certain operating conditions that may be objectionable for some applications. This paper develops an analytical model that captures the coupled structural dynamic, acoustic and heat transfer physics, but is also simple enough to investigate the underlying physical behavior of the parameters that govern the jet’s performance and run many trade-o studies. Detailed comparison with experimental results is also discussed. Despite issues with the comparison of some parameters impacting jet performance, such as disk velocity and exit velocity, the predicted sound intensity and heat transfer coecient agree well with experimental test data.

Computational Modeling and Analysis of Biomimetic Flight Mechanisms

Numerical simulations have been used to study the fluid dynamics associated with flapping flight modes of single as well as paired wings. The numerical solver employed is based on a sharp-interface, Cartesian grid method that allows us to simulate flow with moving boundaries on stationary Cartesian grids. Here, the simple case of normal hover is examined and the thrust production and vortex dynamics of a single wing undergoing a combined pitch-and-heave maneuver is analyzed. Following this, the fluid flow associated with a paired wing is examined with particular emphasis on the effect of phase lag between the two wings on thrust production and efficiency.

Fluid-Structure Interaction Model for Low-Frequency Synthetic Jets

Advancements in microelectronics have increased circuit board heat densities to the point where active cooling is required. Synthetic jets offer interesting capabilities for localized active cooling of electronics due to their compact size, low cost, and substantial cooling effectiveness. The design of synthetic jets for specific applications requires practical design tools that capture the strong fluid-structure interaction without computationally long run times. There is particular interest in synthetic jets that have a low operating frequency to reduce noise levels. This paper describes how common finite elements and codes can be used to calculate parameters for a synthetic jet fluid- structure interaction model that only requires a limited number of degrees of freedom and is solved using a direct approach for low-frequency synthetic jets. Extensive tests are performed with the synthetic jet in vacuum to measure deflection, in ambient air to measure pressure and exit velocity, and impinging on a heated surface to measure heat transfer enhancement. The test results are compared with the fluid-structure interaction model results for validation, and agreement is found to be good in the frequency range of interest from 200 to 500 Hz.

Accurate time-dependent computations and reduced-order modeling for multiphase flows

In the present study, we initiate development of a non-iterative multiphase algorithm by enhancing the Pressure Implicit with Splitting of Operators (PISO) algorithm. The Gallium fusion problem, which is characterized by a solid-liquid phase front and natural convection effects, is employed as a test case for validation. The problem poses serious computational issues in form of a non-linear energy equation and a strong pressure-velocity-temperature coupling. The single-fluid modeling approach is adopted in conjunction with the enthalpy-based formulation for the temperature equation. The current algorithm computes the solution through a series of predictor-corrector steps with special treatment to achieve rapid convergence of the energy equation. The algorithm demonstrates an enhanced performance for the highly unsteady, chosen test problem. A reduced-order analysis of the simulated data is also performed by Proper Orthogonal Decomposition (POD). Specifically, impact of the constantly changing flow domain, and flow scales, on the POD implementation is highlighted.Copyright