Christian Navid Nayeri

Steady and Unsteady Plasma Wall Jets for Separation and Circulation Control

An experimental investigation of separation and circulation control was carried out using corona discharge as well as dielectric barrier discharge actuators at typical micro air vehicle (MAV) Reynolds numbers. All actuators were calibrated by direct measurement and their limitations were assessed on the basis of conventional low Reynolds number active flow control data. Aerodynamic data from corona discharge and high frequency dielectric barrier discharge actuators highlighted their applicability at MAV-type Reynolds numbers. Modulating the dielectric barrier discharge actuators at frequencies corresponding to reduced frequencies of O(1), resulted in significant improvements to Cl,max, which increased with decreasing Re. At the low end of the MAV Reynolds number range (Re~20,000) modulation increased Cl,max by more than a factor of 2 and typical low Re hysteresis was eliminated. Of particular interest from an applications perspective was that performance, measured here by Cl,max, was shown to increase with decreasing duty cycle, and hence power input. In fact, duty cycles of around 0.66% were sufficient for effective separation control, corresponding to power inputs on the order of 1.2 milliwatts per centimeter.

Experimental Study on Bluff Body Drag Reduction with Fluidic Oscillators

In this study fluidic oscillators are examined for separation control purposes over base flaps attached to a three-dimensional bluff body which resembles the rectangular shape of a tractor-trailer model. Fluidic oscillators continuously emit a high velocity jet which is spatially oscillating along the flaps’ span at high frequency. These flow control actuators successfully prevent flow separation over the base flaps, thereby significantly reducing the model’s drag. Even when accounting for an overestimated momentum input, the total drag is reduced by 16% without any system or setup optimization. It was found that the drag reduction improves when increasing the flaps’ length at a constant actuation level. Furthermore, a larger spacing between adjacent actuators reduces the minimal momentum input required to attach the flow. Instead of the momentum coefficient, the velocity ratio governs the actuation intensity for changing actuator spacing. A velocity ratio of five yields the most efficient results. Some qualitative flow field observations demonstrate the actuators’ effectiveness in preventing flow separation over the entire flap. Additionally, vortical structures are observed in the vicinity of the jets’ exit, which are suggested to be the reason for the oscillators’ superior performance.

Control of Thick Airfoil, Deep Dynamic Stall Using Steady Blowing

The utility of constant blowing as an aerodynamic load control concept for wind turbine blades was explored experimentally. A NACA 0018 airfoil model equipped with control slots near the leading edge and at mid-chord was investigated initially under quasi-static conditions at Reynolds numbers ranging from 1.25·105 to 3.75·105. Blowing from the leading-edge slot showed a significant potential for load control applications. Leading-edge stall was either promoted or inhibited depending on the momentum coefficient, and a corresponding reduction or increase in lift on the order of Δcl≈0.5 was obtained. Control from the mid-chord slot counteracted trailing-edge stall but was ineffective at preventing leading-edge separation. The impact of blowing from the leading-edge slot on dynamic stall was explored by means of unsteady surface pressure measurements and simultaneous particle image velocimetry above the suction surface. At a sufficiently high momentum coefficient, the formation and shedding of the dynamic sta...

Phase-Averaging Methods for the Natural Flowfield of a Fluidic Oscillator

The presented study examines various methods for phase averaging the naturally oscillating flowfield of a scaled-up fluidic oscillator. No external trigger is employed to control the oscillation of the flow. Mathematical and signal conditioning approaches for phase averaging the data are categorized and described. The results of these methods are evaluated for their accuracy in capturing the natural flowfield. The respective criteria are based on the minimum fluctuation in oscillation period length, the conservation of velocity amplitudes, and the number of snapshots per phase-averaging window. Although all methods produce reasonable qualitative results, only two methods are identified to provide the desired quantitative accuracy and suitability for the investigated flowfield. The first method is based on conditioning a time-resolved pressure signal from the feedback channels in the oscillator. An autocorrelation applied to the reference signal improves the period identification. The second method employs...

Extracting quantitative data from tuft flow visualizations on utility scale wind turbines

First results of a novel measurement technique that allows to extract quantitative data from tuft flow visualizations on real-world wind turbine blades are presented. The instantaneous flow structure is analyzed by tracking individual flow indicators in each of the snapshot images. The obtained per-tuft statistics are correlated with logged turbine data to provide an insight into the surface flow structure under the influence of wind speed. A histogram filter is used to identify two flow states: a separated flow state that occurs at higher wind speeds and a maximal attached flow state that mainly occurs in the lower wind speed range.

Vortex Generators for Wind Turbine Blades: A Combined Wind Tunnel and Wind Turbine Parametric Study

Vortex generators (VGs) are passive flow control devices commonly employed to prevent flow separation on wind turbine blades. They mitigate the damaging fatigue loads resulting from stall while increasing lift and consequently lead to rotor torque increase. This work summarizes a research project aimed at optimizing the sectional as well as the full rotor-blade aerodynamics using VGs.The effects of chordwise position, spanwise spacing and VG size were studied with force balance measurements of a 2D wing section. Reducing the distance between adjacent VGs produced large increases in the static stall angle and maximum lift, but also resulted in a significant increase in drag as well as sharp lift excursions at angles exceeding the static stall angle. The optimal chordwise position of the vortex generators was found to be in the range of x/c = 15%–20%, where a comparatively low parasitic drag and a smooth post-stall lift curve were achieved. Particle Image Velocimetry measurements were conducted at various chordwise positions to provide insight into the interaction between adjacent streamwise vortices.The experimental aerodynamic performance curves of the optimal VG configuration were used to project their effect on wind turbine blade aerodynamics. Three different rotorblades were designed and several stall and pitch regulated wind turbine models were simulated by means of a Blade Element Momentum (BEM) code (QBlade) developed by Smart Blade GmbH. The performance of the rotorblades with and without VGs was simulated in order to assess their effect on the aerodynamic performance and loads. Finally, previously measured steady state performance curves under high-roughness conditions were used to simulate the detrimental effect of surface roughness on the performance of the aforementioned rotorblades. This allows for an estimate of the potential of the VGs to be employed as retrofit elements for performance recovery of blades with a contaminated surface.© 2012 ASME

Plasma Flow Control on Low Aspect Ratio Wings at Low Reynolds Numbers

Results of an ongoing study on flow control on low aspect ratio wings are presented. The Reynolds number regime Re < 100 000 is aimed at micro air vehicle (MAV) applications. Force measurements for different wing–actuator combinations are compared and the frequency response of lift increase at different angles of attack is discussed. A frequency nondimensionalization for active flow control on low aspect ratio wings inspired by bluff body aerodynamics is proposed to make the frequency scan data independant of angle of attack. To gain insight into the changes of flowfield characteristics as control is applied both time mean and phase averaged PIV data are presented and discussed.

Pulsed Plasma Actuators for Active Flow Control at MAV Reynolds Numbers

An experimental investigation of separation control using steady and pulsed plasma actuators was carried out on an Eppler E338 airfoil at typical micro air vehicle Reynolds numbers (20,000≤Re≤140,000). Pulsing was achieved by modulating the high frequency plasma excitation voltage. The actuators were calibrated directly using a laser doppler anemometer, with and without free-stream velocity, and this allowed the quantification of both steady and unsteady momentum introduced into the flow. At conventional low Reynolds numbers (Re>100,000) asymmetric single phase plasma actuators can have a detrimental effect on airfoil performance due to the introduction of low momentum fluid into the boundary layer. The effect of modulation, particularly at frequencies corresponding to F +=1, became more effective with decreasing Reynolds number resulting in significant improvements in C L,max. This was attributed to the increasing momentum coefficient, which increased as a consequence of the decreasing free-stream velocities. Particularly low duty cycles of 3% were sufficient for effective separation control, corresponding to power inputs on the order of 5 milliwatts per centimeter.

Three-dimensional mixing layers and their relatives

Abstract In this paper a survey of three-dimensional features in free shear flows and in wall bounded turbulent flows is presented. An attempt is made to develop a classification for three-dimensional shear flows. Some particular configurations of turbulent mixing layers are discussed in detail including concepts of flow control and their application.

Drag Reduction on a Generic Tractor-Trailer using Active Flow Control in Combination with Solid Flaps

An experimental investigation was carried out to assess the drag reducing potential of active flow control in conjunction with flat panel flaps attached to the trailer of a generic tractor–trailer model. The experiments were carried out in a wind tunnel with a 1/10th scale generic tractor-trailer model at Reynolds numbers up to 640,000 based on the model width. Active flow control was achieved by means of constant blowing, constant suction and oscillatory blowing and suction. A secondary objective was to make short base flaps with active flow control as effective as long flaps with no active flow control. Measurement techniques such as flow visualizations, loads by means of a 6-component balance, LDA and PIV were employed. The results show that constant blowing at a momentum coefficient of 11.13% is able to achieve higher drag reduction than long flaps with no active flow control. The analysis of the flow field in the wake showed that constant blowing deflects the shear layer between the free stream and the wake region downward and hence reduces the size of the wake. The flaps at the side of the truck did not appear to have any substantial drag reducing effect.