Avraham Seifert

Suction and Pulsed-Blowing Flow Control Applied to an Axisymmetric Body

A flow-control study using steady suction and pulsed blowing in close proximity was conducted on an axisymmetric bluff body at length-based Reynolds numbers between 1.0 and 4.0×106. The study included a coupled incremental computational-fluid-dynamics and experimental approach. It began with computations of various model setup designs. Subsequently, flow-control experiments and computations were used to optimize steady suction alone. Finally, flow control was provided by a synchronized array of 28 suction and oscillatory blowing actuators, positioned slightly upstream of the baseline separation. Results show suction alone has a limited ability to delay separation and reduce drag on this geometry. Suction located far from the baseline separation is shown to actually increase drag. Addition of pulsed blowing enables separation delay to the trailing edge and drag to be nullified. Increased overall system efficiency, including estimated total actuator power invested, was found at low momentum input for optima...

Nonlinear electrokinetic phenomena around nearly insulated sharp tips in microflows

This contribution seeks to provide for the first time a combined comprehensive theoretical prediction and quantitative experimental (microparticle imaging velocimetry--micro-PIV) measurements of the nonlinear electrokinetic flow around sharp tips in order to substantiate former theoretical and qualitative experimental flow visualization results [S.K. Thamida, H.C Chang, Phys. Fluids 14 (2002) 12; P. Takhistov, K. Duginova, H.C. Chang, J. Colloid Interface Sci. 263 (2003) 133; G. Yossifon, I. Frankel, T. Miloh, Phys. Fluids 18 (2006) 117108]. The study focuses on two microchannel designs: an L-shaped channel and two isolated tips in a straight channel, important in engineering for mixing and particle-trapping purposes. The new experimental results were explained in terms of an induced-charge electrokinetic mechanism alone, without the concentration polarization mechanism as suggested by earlier studies. The vortex generation phenomenon around corners was explained in terms of the varying ratio between the equilibrium and the induced-charge zeta-potentials, showing fair qualitative agreement between numerical and experimental results. Hence, a transition from an irrotational to nonlinear-dominated flow with a vortex pattern occurs beyond a certain electric-field threshold. In particular, for the L-shaped channel case, it is demonstrated that beyond a second field threshold an upstream vortex appears in addition to the downstream one.

Simplified dragonfly airfoil aerodynamics at Reynolds numbers below 8000

Effective aerodynamics at Reynolds numbers lower than 10 000 is of great technological interest and a fundamental scientific challenge. The current study covers a Reynolds number range of 2000–8000. At these Reynolds numbers, natural insect flight could provide inspiration for technology development. Insect wings are commonly characterized by corrugated airfoils. In particular, the airfoil of the dragonfly, which is able to glide, can be used for two-dimensional aerodynamic study of fixed rigid wings. In this study, a simplified dragonfly airfoil is numerically analyzed in a steady free-stream flow. The aerodynamic performance (such as mean and fluctuating lift and drag), are first compared to a “traditional” low Reynolds number airfoil: the Eppler-E61. The numerical results demonstrate superior performances of the corrugated airfoil. A series of low-speed wind and water tunnel experiments were performed on the corrugated airfoil, to validate the numerical results. The findings indicate quantitative agree...

Drag-Reduction Mechanisms of Suction-and-Oscillatory-Blowing Flow Control

An active-flow-control study, using steady suction-and-oscillatory-blowing actuators, was conducted on an axisymmetric bluff-body model for a range of Reynolds numbers between 2×106 and 5×106. Prev...

On Amplitude Scaling of Active Separation Control

Various scaling options for the effects of excitation magnitude on the lift alternation due to zero-mass-flux periodic excitation for boundary layer separation control are examined. Physical scaling analysis leads to five amplitude parameters. The different scaling laws are examined using experimental data acquired at low Reynolds numbers and various angles of attack. The results indicate that both the velocity ratio and the momentum coefficient, commonly used for amplitude scaling of separation control applications, do not scale the current data-set. For 2D excitation with a Strouhal number of order unity, a Reynolds weighted momentum coefficient provides reasonable scaling. For 3D excitation with a Strouhal number greater than 10, the Reynolds scaled momentum coefficient, the Strouhal scaled velocity ratio and the newly defined vorticity-flux coefficient, all provide good scaling. The airfoil incidence variations are accounted for by using the velocity at the boundary layer edge at the actuation location, rather than the fixed free-stream velocity as a velocity scale. The main finding of this study is that the Reynolds number scaled momentum coefficient provides good amplitude scaling for the entire current data set.

Active Flow Control and Part-Span Slat Interactions

According to current art, high-lift devices such as leading-edge slats are essential during the takeoff and landing stages of large transport planes. Because of complexity of the mechanical positioning mechanism and of the specific geometry, there are regions on the main wing that are significantly affected by the slat edge and associated (though not resolved in this study) tip vortices. Under adverse conditions, the vortices degrade the exposed region of the wing performance, which results in lower lift and higher drag. In this experimental study, an airfoil with leading-edge slat was tested for performance enhancement of a thick airfoil section with and without operating active flow control system positioned at the 20% chord location. It was found that the slat improved the overall performance and that the active flow control alone can deliver similar performance gains as the slat for this type of wing section with achievable low actuator control input. Part-span slat configurations, covering 31.8 and 7...

Evaluation Criteria and Performance Comparison of Actuators

Active flow control (AFC) relies on actuators’ control authority as a primary enabling technology for flow manipulation. The requirements of the actuation systems are revisited and tools for critical evaluation of actuators are offered. The application in mind is boundary layer separation control. To be accepted by industry, system efficiency should always be assessed, not only the improvement in aerodynamic performance. Clearly there are additional considerations. A few performance based criteria for comparing different actuation concepts are offered, considering the actuation power and system weight. Relevant recent separation control data are compared and discussed.

Distributed Closed-Loop Lift Control for Performance Recovery of a Thick Turbulent Airfoil

The experiment was aimed at recovering performance of a thick, turbine blade airfoil degraded due to poor surface quality at Reynolds number of around 500,000. A closed-loop system that controls lift is presented. We used up to three rows of “synthetic jets” coupled with an array of time resolved hot film and pressure sensors to estimate the separation location and lift. Using an amplitude distribution algorithm it was possible to recover the clean turbine blade performance and change the blades lift force as desired over a range of working conditions.

On the application of active flow control to wind turbines

This paper reviews recent studies conducted at Tal-Aviv University that were aimed at applying active flow control technology to large and small scale wind turbines with the purpose of improved performance and reduced noise emission. Large wind turbines suffer from degradation of surface quality over the 20 years life span. Premature transition results and promotes early turbulent separation. Energizing the boundary layer with vortex generators (VG’s) and active flow control is studied. While both methods are capable of recovering lost performance, VG’s have an off-design drag penalty, while AFC can be turned off when not needed. It is demonstrated that AFC is highly energy efficient. Small wind turbines suffer from low Reynolds number effects resulting from low wind speed, small chord and low tip speed ratio. Passive and semi-active flow control methods are applied to a small vertical axis wind turbine (VAWT) with the purpose of delaying boundary layer separation via forced transition and transient semi-active fluidic jets. Power extraction at low wind speeds is demonstrated to be greatly improved.

Exploiting the Confined Twin-Jet Instability for Micro Mixing Enhancement

A method for exploiting hydrodynamic instability for enhancement of micro-mixing is proposed, studied numerically and demonstrated experimentally. The confined twin-jet geometry is selected as the baseline unstable flow, since it undergoes a Hopf bifurcation at low Reynolds numbers. It is shown that by adding weak fluidic perturbation to the mean flow, significant amplification of unstable modes results, leading to enhanced mixing in laminar flow. Good agreement was found between Computational Fluid Dynamics (CFD) and experimental results for the critical Re, for which instability sets-in, and its related Struohal number. In order to evaluate mixing efficiency, backward tracing particle was performed using the computed flow field. Mixing is estimated based on particle’s location in time and space. A parametric mixing study was performed and its results are presented and discussed. It is shown that mixing was significantly enhanced when the perturbation frequency was matched with the baseline’s most-unstable Struohal number, even for sub-critical Reynolds number.Copyright