Longitudinal development of flow-separation lines on slender bodies in translation

2018 ◽  
Vol 837 ◽  
pp. 627-639 ◽  
Author(s):  
S.-K. Lee
Keyword(s):  
Reynolds Number ◽  
Power Law ◽  
Flow Separation ◽  
Incidence Angle ◽  
The Body ◽  
Flow Visualisation ◽  
Start Time ◽  
Separating Flow ◽  
Primary Separation ◽  
Axisymmetric Bodies

This paper examines flow-separation lines on axisymmetric bodies with tapered tails, where the separating flow takes into account the effect of local body radius $r(x)$, incidence angle $\unicode[STIX]{x1D713}$ and the body-length Reynolds number $\mathit{Re}_{L}$. The flow is interpreted as a transient problem which relates the longitudinal distance $x$ to time $t^{\ast }=x\,\text{tan}(\unicode[STIX]{x1D713})/r(x)$, similar to the approach of Jeans & Holloway (J. Aircraft, vol. 47 (6), 2010, pp. 2177–2183) on scaling separation lines. The windward and leeward sides correspond to the body azimuth angles $\unicode[STIX]{x1D6E9}=0$ and $180^{\circ }$, respectively. From China-clay flow visualisation on axisymmetric bodies and from a literature review of slender-body flows, the present study shows three findings. (i) The time scale $t^{\ast }$ provides a collapse of the separation-line data, $\unicode[STIX]{x1D6E9}$, for incidence angles between $6$ and $35^{\circ }$, where the data fall on a power law $\unicode[STIX]{x1D6E9}\sim (t^{\ast })^{k}$. (ii) The data suggest that the separation rate $k$ is independent of the Reynolds number over the range $2.1\times 10^{6}\leqslant \mathit{Re}_{L}\leqslant 23\times 10^{6}$; for a primary separation $k_{1}\simeq -0.190$, and for a secondary separation $k_{2}\simeq 0.045$. (iii) The power-law curve fits trace the primary and secondary lines to a characteristic start time $t_{s}^{\ast }\simeq 1.5$.

The flow structure in the lee of an inclined 6:1 prolate spheroid

1994 ◽  
Vol 269 ◽  
pp. 79-106 ◽  
Author(s):  
T. C. Fu ◽  
A. Shekarriz ◽  
J. Katz ◽  
T. T. Huang
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Flow Structure ◽  
Incidence Angle ◽  
Prolate Spheroid ◽  
The Body ◽  
Boundary Layer Transition ◽  
Primary Vortex ◽  
Vortex Sheets ◽  
Vorticity Distribution

Particle displacement velocimetry is used to measure the velocity and vorticity distributions around an inclined 6: 1 prolate spheroid. The objective is to determine the effects of boundary-layer tripping, incidence angle, and Reynolds number on the flow structure. The vorticity distributions are also used for computing the lateral forces and rolling moments that occur when the flow is asymmetric. The computed forces agree with results of direct measurements. It is shown that when the flow is not tripped, separation causes the formation of a pair of vortex sheets. The size of these sheets increases with increasing incidence angle and axial location. Their orientation and internal vorticity distribution also depend on incidence. Rollup into distinct vortices occurs in some cases, and the primary vortex contains between 20 % and 50 % of the overall circulation. The entire flow is unsteady and there are considerable variations in the instantaneous vorticity distributions. The remainder of the lee side, excluding these vortex sheets, remains almost vorticity free, providing clear evidence that the flow can be characterized as open separation. Boundary-layer tripping causes earlier separation on part of the model, brings the primary vortex closer to the body, and spreads the vorticity over a larger region. The increased variability in the vorticity distribution causes considerable force fluctuations, but the mean loads remain unchanged. Trends with increasing Reynolds number are conflicting, probably because of boundary-layer transition. The separation point moves towards the leeward meridian and the normal force decreases when the Reynolds number is increased from 0.42 × 106 to 1.3 × 106. Further increase in the Reynolds number to 2.1 × 106 and tripping cause an increase in forces and earlier separation.

Nonlinear Computation of Water Impact of Axisymmetric Bodies

2011 ◽  
Vol 55 (01) ◽  
pp. 29-44
Author(s):  
Hongmei Yan ◽  
Yuming Liu
Keyword(s):  
Free Surface ◽  
Flow Separation ◽  
Impact Force ◽  
Surface Profile ◽  
The Body ◽  
Gravity Effect ◽  
Water Impact ◽  
Free Surface Profile ◽  
Axisymmetric Bodies ◽  
The Impact

A fully nonlinear numerical simulation based on a boundary element method was used to investigate water impact of axisymmetric bodies that strike vertically the horizontal free surface from the air. The main objective was to understand the gravity effect on flow/wave kinematics and dynamics and to quantify the range of validity of existing theories and computations that are based on the infinite Froude number assumption. Two body geometries were considered: inverted cone and sphere. For the inverted cone, we obtained detailed dependencies of free-surface profile and impact pressure and load on the body on the generalized Froude number (Fr(V/gt)1/2, where V is the impact velocity, g is the gravitational acceleration, and t is time) and deadrise angle a. Based on these, we developed an approximate formula for evaluating the contribution of the gravity effect to the total impact force on the body in terms of a similarity parameter Fr/a1/2. For the sphere, we developed and applied a pressure-based criterion to follow the evolution of flow separation on the body and to obtain an appropriate description of the free-surface profile near the body and accurate evaluation of the impact pressure and load on the body during the entire impact process. The numerical result of impact force on the body agreed well with existing experimental measurements. We confirmed that the gravity effect is unimportant in initial impact of the sphere. Significantly, we found that in a later stage of impact, flow separation remains at an almost fixed position at an angle u 62.5 deg to the bottom of the sphere for a wide range of Froude numbers, Fr V/(gR)1/2 1, where R is the radius of the sphere.

scholarly journals Experimental investigation of the flow structures over a 40° swept wing

2019 ◽  
Vol 123 (1259) ◽  
pp. 39-55 ◽  
Author(s):  
S. Zhang ◽  
A. J. Jaworski ◽  
S. C. McParlin ◽  
J. T. Turner
Keyword(s):  
Reynolds Number ◽  
Incidence Angle ◽  
Leading Edge ◽  
Flow Structures ◽  
Flow Visualisation ◽  
Swept Wing ◽  
Still Images ◽  
Wing Model ◽  
Force Moment ◽  
Flow Vortex

ABSTRACTFlow over a moderately swept wing is characterised by complex localised flow vortex topologies such as ‘closed’ separation bubbles or ‘open’ separation structures. A model of a complex cambered, twisted, tapered wing with 40° leading edge sweep, representative of those designed for manoeuvre at high subsonic Mach numbers, was investigated using the oil-film visualisation, stereo particle image velocimetry and force moment measurements. Wind-tunnel tests were conducted at a range of Reynolds number from 2.1×105 to 8.4×105 and at angles of incidence from −1° to 22°. Still images combined with video clips enable flow patterns over the wing model to be interpreted more clearly and accurately. Using successive images extracted from the video of flow visualisation, the movement of the oil pigment has been estimated. The influence of the Reynolds number and incidence angle was discussed through analysing the flow pattern over the wing surface. Additionally, the link between the flow structures present and the wing aerodynamic performance was studied.

Transition delay by surface heating: a zonal analysis for axisymmetric bodies

2000 ◽  
Vol 418 ◽  
pp. 77-100 ◽  
Author(s):  
RAMA GOVINDARAJAN ◽  
R. NARASIMHA
Keyword(s):  
Reynolds Number ◽  
Transition Zone ◽  
Stability Theory ◽  
Axisymmetric Body ◽  
The Body ◽  
Linear Stability Theory ◽  
Heated Body ◽  
Flow Reynolds Number ◽  
Transition Delay ◽  
Axisymmetric Bodies

It has long been known from linear stability theory that heating a surface immersed in water flow tends to stabilize the boundary layer on the surface, suggesting that there may be a corresponding delay in transition. Experiments confirm the suggestion, but based on intermittency data on a heated body of revolution (Lauchle & Gurney 1984) it has been inferred that incremental changes in transition Reynolds number diminish as the overheat increases. The parameter chosen to locate transition in the analysis leading to this conclusion corresponds to the point where the intermittency is 0.5. However, intermittency distributions in the transition zone on an axisymmetric body may contain what have been called ‘subtransitions’ (Narasimha 1984). Taking this possibility into account, we formulate here a model for the variation of intermittency with flow Reynolds number at a fixed station on the body, as in the experiments. The rate at which turbulent spots merge with each other is shown to determine the location of subtransition. The transition onset Reynolds number (corresponding to the location where intermittency begins to depart from zero), inferred from the data on the basis of this model, shows a continuing increase with the temperature overheat, a trend in closer agreement with stability theory; but the axisymmetric body geometry results in a very short transition zone, countering in part the benefits of transition delay.

scholarly journals Numerical analysis of high-lift configurations with oscillating flaps

2021 ◽  
Author(s):  
Johannes Ruhland ◽  
Christian Breitsamter
Keyword(s):  
Reynolds Number ◽  
Flow Separation ◽  
Separated Flow ◽  
Navier Stokes ◽  
Rotational Axis ◽  
High Lift ◽  
Hinge Point ◽  
Reduced Frequency ◽  
Flap Deflection ◽  
The Impact

AbstractThis study presents two-dimensional aerodynamic investigations of various high-lift configuration settings concerning the deflection angles of droop nose, spoiler and flap in the context of enhancing the high-lift performance by dynamic flap movement. The investigations highlight the impact of a periodically oscillating trailing edge flap on lift, drag and flow separation of the high-lift configuration by numerical simulations. The computations are conducted with regard to the variation of the parameters reduced frequency and the position of the rotational axis. The numerical flow simulations are conducted on a block-structured grid using Reynolds Averaged Navier Stokes simulations employing the shear stress transport $$k-\omega $$ k - ω turbulence model. The feature Dynamic Mesh Motion implements the motion of the oscillating flap. Regarding low-speed wind tunnel testing for a Reynolds number of $$0.5 \times 10^{6}$$ 0.5 × 10 6 the flap movement around a dropped hinge point, which is located outside the flap, offers benefits with regard to additional lift and delayed flow separation at the flap compared to a flap movement around a hinge point, which is located at 15 % of the flap chord length. Flow separation can be suppressed beyond the maximum static flap deflection angle. By means of an oscillating flap around the dropped hinge point, it is possible to reattach a separated flow at the flap and to keep it attached further on. For a Reynolds number of $$20 \times 10^6$$ 20 × 10 6 , reflecting full scale flight conditions, additional lift is generated for both rotational axis positions.

scholarly journals Movement of a finite body in channel flow

2016 ◽  
Vol 472 (2191) ◽  
pp. 20160164 ◽  
Author(s):  
Frank T. Smith ◽  
Edward R. Johnson
Keyword(s):  
Reynolds Number ◽  
Channel Flow ◽  
Flow Instability ◽  
Flow Direction ◽  
Finite Size ◽  
The Body ◽  
Thin Body ◽  
Shear Stresses ◽  
Unsteady Interaction ◽  
High Reynolds

A body of finite size is moving freely inside, and interacting with, a channel flow. The description of this unsteady interaction for a comparatively dense thin body moving slowly relative to flow at medium-to-high Reynolds number shows that an inviscid core problem with vorticity determines much, but not all, of the dominant response. It is found that the lift induced on a body of length comparable to the channel width leads to differences in flow direction upstream and downstream on the body scale which are smoothed out axially over a longer viscous length scale; the latter directly affects the change in flow directions. The change is such that in any symmetric incident flow the ratio of slopes is found to be cos ⁡ ( π / 7 ) , i.e. approximately 0.900969, independently of Reynolds number, wall shear stresses and velocity profile. The two axial scales determine the evolution of the body and the flow, always yielding instability. This unusual evolution and linear or nonlinear instability mechanism arise outside the conventional range of flow instability and are influenced substantially by the lateral positioning, length and axial velocity of the body.

scholarly journals Experimental Investigation of Tandem Blade Cascades With Double-Circular ARC Profiles

1985 ◽  
Author(s):  
Wu Guochuan ◽  
Zhuang Biaonan ◽  
Guo Bingheng
Keyword(s):  
Experimental Investigation ◽  
Reynolds Number ◽  
Mach Number ◽  
Total Pressure ◽  
Incidence Angle ◽  
Axial Direction ◽  
Geometrical Parameters ◽  
Optimal Conditions ◽  
Turning Angle ◽  
Circular Arc

24 double circular are tandem blade cascades of three different chord-ratios were investigated under different displacements in peripheral and axial direction. The inlet Mach number was 0.3. The Reynolds number based on blade chord was 2.7×105. The characteristics of the tandem blade cascades, such as the dependence of turning angle and coefficient of total pressure loss on incidence angle were obtained. The ranges of main geometrical parameters under optimal conditions were recommended.

Rotating elliptic cylinders in a viscous fluid at rest or in a parallel stream

1977 ◽  
Vol 79 (1) ◽  
pp. 127-156 ◽  
Author(s):  
Hans J. Lugt ◽  
Samuel Ohring
Keyword(s):  
Reynolds Number ◽  
Fluid Flow ◽  
Numerical Solutions ◽  
Elliptic Cylinder ◽  
The Body ◽  
Oscillatory Behaviour ◽  
Incompressible Fluid Flow ◽  
Transient Period ◽  
Parallel Stream ◽  
Rotating Bodies

Numerical solutions are presented for laminar incompressible fluid flow past a rotating thin elliptic cylinder either in a medium at rest at infinity or in a parallel stream. The transient period from the abrupt start of the body to some later time (at which the flow may be steady or periodic) is studied by means of streamlines and equi-vorticity lines and by means of drag, lift and moment coefficients. For purely rotating cylinders oscillatory behaviour from a certain Reynolds number on is observed and explained. Rotating bodies in a parallel stream are studied for two cases: (i) when the vortex developing at the retreating edge of the thin ellipse is in front of the edge and (ii) when it is behind the edge.

Plasma actuation effect on a NACA 4412 airfoil

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Burak Karadag ◽  
Cem Kolbakir ◽  
Ahmet Selim Durna
Keyword(s):  
Reynolds Number ◽  
Flow Separation ◽  
Open Loop ◽  
Electrode Configuration ◽  
Small Scale ◽  
Low Velocity ◽  
Dbd Plasma ◽  
Content Type ◽  
Plasma Actuation ◽  
Electric Wind

Purpose This paper aims to investigate the effects of a dielectric barrier discharge (DBD) plasma actuator (PA) qualitatively on aerodynamic characteristics of a 3 D-printed NACA 4412 airfoil model. Design/methodology/approach Airflow visualization study was performed at a Reynolds number of 35,000 in a small-scale open-loop wind tunnel. The effect of plasma actuation on flow separation was compared for the DBD PA with four different electrode configurations at 10°, 20° and 30° angles of attack. Findings Plasma activation may delay the onset of flow separation up to 6° and decreases the boundary layer thickness. The effects of plasma diminish as the angle of attack increases. Streamwise electrode configuration, in which electric wind is produced in a direction perpendicular to the freestream, is more effective in the reattachment of the airflow compared to the spanwise electrode configuration, in which the electric wind and the free stream are in the same direction. Practical implications The Reynolds number is much smaller than that in cruise aircraft conditions; however, the results are promising for low-velocity subsonic airflows such as improving control capabilities of unmanned aerial vehicles. Originality/value Superior efficacy of spanwise-generated electric wind over streamwise-generated one is demonstrated at a very low Reynolds number. The results in the plasma aerodynamics literature can be reproduced using ultra-low-cost off-the-shelf components. This is important because high voltage power amplifiers that are frequently encountered in the literature may be prohibitively expensive especially for resource-limited university aerodynamics laboratories.

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