Active Control of Flow Separation in a Radial Blower

2010 ◽  
Vol 132 (5) ◽  
Author(s):  
David Greenblatt ◽  
Guy Arzuan
Keyword(s):  
Pressure Rise ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Input Power ◽  
Maximum Pressure ◽  
Proof Of Concept ◽  
Periodic Excitation ◽  
Separated Shear Layer ◽  
Reduced Frequency ◽  
Blower Performance

An experimental investigation was undertaken as a proof-of-concept study for active separation control in a radial blower. Acoustic perturbations were introduced into the impeller housing of a small radial blower with fully stalled blades. Increases in the plenum pressure of 35% were achieved and, based on tuft-based flow visualization, it was concluded that the pressure increases were brought about due to excitation and deflection of the leading-edge separated shear layer. Within the parameter range considered here, the optimum dimensionless control frequencies were found to be O(0.5), irrespective of the blade orientation or number of blades. Moreover, the maximum pressure rise was achieved with an investment of only 2% of the fan input power. Backward bladed impeller blades exhibited slightly larger increases in pressure coefficients when compared with their forward bladed counterparts. The dependence of blower performance on reduced frequency was remarkably similar to that seen on flat plate airfoils at similar Reynolds numbers under periodic excitation.

Three-Dimensional Vortex Structure in a Leading-Edge Separation Bubble at Moderate Reynolds Numbers

1991 ◽  
Vol 113 (3) ◽  
pp. 405-410 ◽  
Author(s):  
Kyuro Sasaki ◽  
Masaru Kiya
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Plate Thickness ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Vortex Filaments ◽  
Separated Shear Layer ◽  
Hydrogen Bubbles ◽  
Edge Separation

This paper describes the results of a flow visualization study which concerns three-dimensional vortex structures in a leading-edge separation bubble formed along the sides of a blunt flat plate. Dye and hydrogen bubbles were used as tracers. Reynolds number (Re), based on the plate thickness, was varied from 80 to 800. For 80 < Re < 320, the separated shear layer remains laminar up to the reattachment line without significant spanwise distortion of vortex filaments. For 320 < Re < 380, a Λ-shaped deformation of vortex filaments appears shortly downstream of the reattachment and is arranged in-phase in the downstream direction. For Re > 380, hairpin-like structures are formed and arranged in a staggered manner. The longitudinal and spanwise distances of the vortex arrangement are presented as functions of the Reynolds number.

Unsteady thrust, lift and moment of a two-dimensional flapping thin airfoil in the presence of leading-edge vortices: a first approximation from linear potential theory

2018 ◽  
Vol 851 ◽  
pp. 344-373 ◽  
Author(s):  
R. Fernandez-Feria ◽  
J. Alaminos-Quesada
Keyword(s):  
Potential Theory ◽  
Leading Edge ◽  
Input Power ◽  
Linear Potential ◽  
Two Dimensional ◽  
Thin Airfoil ◽  
Reduced Frequency ◽  
Large Phase ◽  
Linear Potential Theory ◽  
Oscillating Foil

The effect of a leading-edge vortex (LEV) on the lift, thrust and moment of a two-dimensional heaving and pitching thin airfoil is analysed within the unsteady linear potential theory. First, general expressions that take into account the effect of any set of unsteady point vortices interacting with the oscillating foil and unsteady wake are derived. Then, a simplified analysis, based on the Brown–Michael model, of the initial stages of the growing LEV from the sharp leading edge during each half-stroke is used to obtain simple expressions for its main contribution to the unsteady lift, thrust and moment. It is found that the LEV contributes to the aerodynamic forces and moment provided that a pitching motion exists, while its effect is negligible, in the present approximation, for a pure heaving motion, and for some combined pitching and heaving motions with large phase shifts which are also characterized in the present work. In particular, the effect of the LEV is found to decrease with the distance of the pivot point from the trailing edge. Further, the time-averaged lift and moment are not modified by the growing LEVs in the present approximation, and only the time-averaged thrust force is corrected, decreasing slightly in most cases in relation to the linear potential results by an amount proportional to$a_{0}^{2}k^{3}$for large$k$, where$k$is the reduced frequency and$a_{0}$is the pitching amplitude. The time-averaged input power is also modified by the LEV in the present approximation, so that the propulsion efficiency changes by both the thrust and the power, these corrections being relevant only for pivot locations behind the midchord point. Finally, the potential results modified by the LEV are compared with available experimental data.

The Effects of Reynolds Number on the Aerodynamic Performance of Geometrically Similar Fans

2008 ◽  
Author(s):  
K. Hanly ◽  
R. Grimes ◽  
P. Walsh
Keyword(s):  
Reynolds Number ◽  
Electronic Device ◽  
Flow Characteristics ◽  
Pressure Rise ◽  
Reynolds Numbers ◽  
Aerodynamic Performance ◽  
Simultaneous Increase ◽  
Maximum Pressure ◽  
Low Profile ◽  
High Reynolds

The cooling of portable electronic devices has become paramount in the last number of years due to the simultaneous increase in power consumption and reduction in package size. This has lead to an increase in the amount of heat that needs to be dissipated by these devices. Passive cooling techniques will no longer provide an adequate solution and therefore active cooling solutions need to be implemented. The use of miniature radial fans in conjunction with heatsinks is a possible solution. These types of fans are especially suited as they can be deployed in a low profile format. However, little is known about the aerodynamic effects of reducing the fan scale and therefore Reynolds number to the extent necessary for use in portable electronic device cooling. This paper looks to quantify deviation of aerodynamic performance with Reynolds number from that predicted by the fan laws. Before tests were carried out experimental facilities were calibrated. Four radial fans with diameters of 80, 40, 18.3 and 10mm were then tested at a number of different rotational speeds with measurements of pressure rise and flow rates recorded for each of these speeds. The measurements presented show the need for a homogonous experimental setup with the exact conditions replicated each time a test is carried out. Results also show that there is good correlation between the experimental results for pressure rise and flow rate at high Reynolds numbers in accordance with trends from high Reynolds number theory. However at the lower Reynolds numbers a fundamental change in flow phenomena emerges which alters the maximum pressure and flow characteristics.

Discrete Vortex Modeling of a Flapping Foil With Activated Leading Edge Motion

2019 ◽  
Author(s):  
Michael W. Prier ◽  
James A. Liburdy
Keyword(s):  
Energy Harvesting ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Time Resolved ◽  
Discrete Vortex ◽  
Suction Parameter ◽  
Flapping Foil ◽  
Vortex Model ◽  
Reduced Frequency ◽  
Effective Angle

Abstract Energy harvesting performance for a flapping foil device is evaluated to determine how activated leading edge motion affects the aerodynamic forces and the cycle power generated. Results are obtained for a thin flat foil that pitches about the mid-chord and operates in the reduced frequency range of k = fc/U of 0.06–0.10 and Reynolds numbers of 20,000 and 30,000 with a pitching amplitude of 70° and heaving amplitude of h0 = 0.5c. Time resolved data are presented based on direct force measurements and are used to determine overall cycle efficiency and coefficient of power. These results are compared against a panel-based discrete vortex model to predict power production. The model incorporates a leading edge suction parameter predictor for vortex shedding and empirical adjustments to circulatory forces. It is found that the leading edge motions that reduce the effective angle of attack early in a flapping stroke generate larger forces later in the stroke. Consequently, the energy harvesting efficiencies and power coefficients are increased since the generated aerodynamic loads are better synchronized with the foil motion. The efficiency gains are reduced with increasing reduced frequencies.

An experimental investigation of a laminar separation bubble on the leading-edge of a modelled aerofoil for different Reynolds numbers

2015 ◽  
Vol 230 (13) ◽  
pp. 2208-2224 ◽  
Author(s):  
A Samson ◽  
S Sarkar
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Separation Bubble ◽  
Experimental Studies ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Constant Thickness ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Separated Shear Layer

This paper describes the dynamics of a laminar separation bubble formed on the semi-circular leading edge of constant thickness aerofoil model. Detailed experimental studies are carried out in a low-speed wind tunnel, where surface pressure and time-averaged velocity in the separated region and as well as in the downstream are presented along with flow field visualisations through PIV for various Reynolds numbers ranging from 25,000 to 75,000 (based on the leading edge diameter). The results illustrate that the separated shear layer is laminar up to 20% of separation length and then the perturbations are amplified in the second half attributing to breakdown and reattachment. The bubble length is highly susceptible to change in Reynolds number and plays an important role in outer layer activities. Further, the transition of a separated shear layer is studied through variation of intermittency factor and comparing with existing correlations available in the literature for attached flow and as well as separated flow. Transition of the separated shear layer occurs through formation of K-H rolls, where the intermittency following spot propagation theory appears valid. The predominant shedding frequency when normalised with respect to the momentum thickness at separation remains almost constant with change in Reynolds number. The relaxation is slow after reattachment and the flow takes about five bubble lengths to approach a canonical layer.

Mean and Fluctuating Velocity Characteristics of a Separated Shear Layer Past a Surface Mounted Block

2006 ◽  
Vol 129 (2) ◽  
pp. 200-208 ◽  
Author(s):  
Ü. Özkol ◽  
C. Wark ◽  
D. Fabris
Keyword(s):  
Shear Layer ◽  
Reynolds Stress ◽  
Reynolds Shear Stress ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Separated Shear Layer ◽  
Block Height ◽  
High Reynolds

The mean velocity, Reynolds stress, and mean vorticity regions of a separated shear layer over a surface mounted block are investigated by 2D Digital Particle Image Velocimetry (DPIV) for three Reynolds numbers (Rea=500, 1000, and 2500) and two channel-to-block height ratios (H∕a=1.825 and 4.6). The recirculation region’s height and length are determined for the separated shear layer by means of U¯=0 contours. It is observed that the high Reynolds stress regions lay just outside of the U¯=0 contours. The flow visualization and DPIV measurement of vorticity indicate that the differing normalized Reynolds stresses between Rea=500 and 1000 are most probably due to the initiation of the vortex shedding between these two Reynolds numbers while, differences are minimal between Rea=1000 and 2500. A sign change in the Reynolds shear stress distribution of the separated shear layer near the leading edge of the block was recognized for every Reynolds number and channel width.

Effects of Elevated Free-Stream Turbulence on Actively Controlled Separation Bubble1

2004 ◽  
Vol 126 (6) ◽  
pp. 1015-1024 ◽  
Author(s):  
E. Halfon ◽  
B. Nishri ◽  
A. Seifert ◽  
I. Wygnanski
Keyword(s):  
Shear Layer ◽  
Free Stream ◽  
Excitation Frequency ◽  
Leading Edge ◽  
Current Control ◽  
Mixing Enhancement ◽  
Destructive Interference ◽  
Periodic Excitation ◽  
Free Stream Turbulence ◽  
Separated Shear Layer

The effects of elevated free-stream turbulence (FST) on natural and periodically excited separation bubbles were examined experimentally at low Reynolds numbers. The bubble was formed at the leading edge of a flat plate and the FST level was altered by placing a grid across the flow at different locations upstream of the plate. The mixing across the separated shear-layer increased due to the elevated FST and the two-dimensional periodic excitation, flattening, and shortening the bubble. Periodic excitation at frequencies that were at least an order of magnitude lower than the initial Kelvin–Helmholtz instability of the separated shear-layer were very effective at low FST. The fundamental excitation frequency and its first harmonic were amplified above the bubble. High frequency excitation (F+≈3, based on the length of the natural low FST bubble that served as a reference baseline) affected the flow near the leading edge of the bubble but it rapidly decayed in the reattachment region. Lower frequencies F+≈1 were less effective and they decayed at a slower rate downstream of reattachment. An increase in FST level reduced the net effect of the periodic excitation on mixing enhancement and subsequent reattachment process. This was probably due to a destructive interference between the nominally 2D excitation and the random, in space and in time, FST. High FST is known to reduce the spanwise coherence in free shear layers [e.g., Chandrasuda, C., Mehta, R. D., Weir, A. D., and Bradshaw, P., 1978, “Effects of free-stream turbulence on large structures in turbulent mixing layers,” J. Fluid Mech., 85, pp. 693–704] and therefore also the effectiveness of the current control strategy, this not withstanding 2D periodic excitation accelerated the reattachment process and the recovery rate of the attached boundary layer, reducing its momentum loss.

Bio-Inspired Wind Energy Harvester

2012 ◽  
Author(s):  
David Talarico ◽  
Kevin Hynes
Keyword(s):  
Energy Harvester ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Flapping Wing ◽  
Oncoming Flow ◽  
Preliminary Testing ◽  
Reduced Frequency ◽  
Effective Operation ◽  
Extraction Device ◽  
Motion Parameters

Engineers and designers often mimic mechanisms found in nature to accomplish a specific task. This concept was applied to wind power generation to simplify and reduce cost of the HAWT design. The advantage of the flapping wing design lies in the additional power generated by leading edge vortices. It has previously been demonstrated by others that a flapping wing section undergoing simultaneous, coordinated heaving and pitching is capable of extracting energy from an oncoming flow using the CFD code FLUENT. The encouraging results suggest that efficiencies as high as 36.3% are achievable with such a system with the strongest influence on efficiency being reduced heaving amplitude, H/c, pitching amplitude, θ, and reduced frequency, f*. The suggested values of these motion parameters for optimal performance was found to be H/c≈1, θ≈73°, and f*≈0.15. Hence, an adjustable flapping wing energy extraction device was designed in accordance with these parameters. Preliminary testing has demonstrated the effective operation of the device over a range of Reynolds numbers in a wind tunnel.

Investigation on the effect of leading edge tubercles of sweptback wing at low reynolds number

2020 ◽  
Vol 21 (6) ◽  
pp. 621
Author(s):  
Veerapathiran Thangaraj Gopinathan ◽  
John Bruce Ralphin Rose ◽  
Mohanram Surya
Keyword(s):  
Leading Edge ◽  
Reynolds Numbers ◽  
Sweep Angle ◽  
Laminar Separation ◽  
Low Reynolds Numbers ◽  
Flow Energy ◽  
Airplane Wing ◽  
Low Reynolds ◽  
Naca 0015 ◽  
Wing Performance

Aerodynamic efficiency of an airplane wing can be improved either by increasing its lift generation tendency or by reducing the drag. Recently, Bio-inspired designs have been received greater attention for the geometric modifications of airplane wings. One of the bio-inspired designs contains sinusoidal Humpback Whale (HW) tubercles, i.e., protuberances exist at the wing leading edge (LE). The tubercles have excellent flow control characteristics at low Reynolds numbers. The present work describes about the effect of tubercles on swept back wing performance at various Angle of Attack (AoA). NACA 0015 and NACA 4415 airfoils are used for swept back wing design with sweep angle about 30°. The modified wings (HUMP 0015 A, HUMP 0015 B, HUMP 4415 A, HUMP 4415 B) are designed with two amplitude to wavelength ratios (η) of 0.1 & 0.24 for the performance analysis. It is a novel effort to analyze the tubercle vortices along the span that induce additional flow energy especially, behind the tubercles peak and trough region. Subsequently, Co-efficient of Lift (CL), Co-efficient of Drag (CD) and boundary layer pressure gradients also predicted for modified and baseline (smooth LE) models in the pre & post-stall regimes. It was observed that the tubercles increase the performance of swept back wings by the enhanced CL/CD ratio in the pre-stall AoA region. Interestingly, the flow separation region behind the centerline of tubercles and formation of Laminar Separation Bubbles (LSB) were asymmetric because of the sweep.

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