Flow Past a Sphere With Surface Blowing and Suction

The effect of uniform surface blowing and suction on the wake dynamics and the drag and lift forces on a sphere is studied using a high-resolution direct numerical simulation technique. The sphere Reynolds number Re, based on its diameter and the freestream velocity, is in the range 1–300. The onset of recirculation in the sphere wake occurs at higher Re, and the transition to nonaxisymmetry and unsteadiness occurs at lower Re in the presence of blowing. The size of the recirculation region increases with blowing, but it nearly disappears in the case of suction. Wake oscillation also increases in the presence of blowing. The drag coefficient in the presence of blowing is reduced compared to that in uniform flow, in the range 10<Re<250, whereas it is increased in the presence of suction. The reduction in the wake pressure minimum associated with the enhanced vortical structures is the primary cause for drag reduction in the case of blowing. In the case of suction, it is the increased surface vorticity associated with the reduction of the boundary layer that results into increased drag. The fluctuations in the instantaneous lift and drag coefficients are significant for blowing, and they result from the asymmetric movement of the wake pressure minimum associated with the shedding process.

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Instability and Transition of a Boundary Layer over a Backward-Facing Step

Fluids ◽

10.3390/fluids7010035 ◽

2022 ◽

Vol 7 (1) ◽

pp. 35

Author(s):

Ming Teng ◽

Ugo Piomelli

Keyword(s):

Boundary Layer ◽

Pressure Gradient ◽

Velocity Distribution ◽

Adverse Pressure Gradient ◽

Transition Process ◽

Transition To Turbulence ◽

Recirculation Region ◽

Backward Facing Step ◽

Freestream Velocity ◽

Displacement Thickness

The development of secondary instabilities in a boundary layer over a backward-facing step is investigated numerically. Two step heights are considered, h/δo*=0.5 and 1.0 (where δo* is the displacement thickness at the step location), in addition to a reference flat-plate case. A case with a realistic freestream-velocity distribution is also examined. A controlled K-type transition is initiated using a narrow ribbon upstream of the step, which generates small and monochromatic perturbations by periodic blowing and suction. A well-resolved direct numerical simulation is performed. The step height and the imposed freestream-velocity distribution exert a significant influence on the transition process. The results for the h/δo*=1.0 case exhibit a rapid transition primarily due to the Kelvin–Helmholtz instability downstream of step; non-linear interactions already occur within the recirculation region, and the initial symmetry and periodicity of the flow are lost by the middle stage of transition. In contrast, case h/δo*=0.5 presents a transition road map in which transition occurs far downstream of the step, and the flow remains spatially symmetric and temporally periodic until the late stage of transition. A realistic freestream-velocity distribution (which induces an adverse pressure gradient) advances the onset of transition to turbulence, but does not fundamentally modify the flow features observed in the zero-pressure gradient case. Considering the budgets of the perturbation kinetic energy, both the step and the induced pressure-gradient increase, rather than modify, the energy transfer.

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Modification of Turbulence in Boundary Layers by Mean and Fluctuating Pressure Gradients

Volume 1: Symposia, Parts A and B ◽

10.1115/fedsm2012-72280 ◽

2012 ◽

Cited By ~ 1

Author(s):

Pranav Joshi ◽

Xiaofeng Liu ◽

Joseph Katz

Keyword(s):

Boundary Layer ◽

Pressure Gradient ◽

Wall Region ◽

Pressure Gradients ◽

Two Dimensional ◽

Time Resolved ◽

Freestream Velocity ◽

Vortical Structures ◽

Fluctuating Pressure ◽

Near Wall

In this study we focus on the effect of mean and fluctuating pressure gradients on the structure of boundary layer turbulence. Two dimensional, time-resolved PIV measurements have been performed upstream of and inside an accelerating sink flow for inlet Reynolds number of Reθ = 3071, and acceleration parameter of K=1.1×10−6. The time-resolved data enables us to calculate the planer projection of pressure gradient by integrating the in-plane components of the material acceleration of the fluid (neglecting out-of-plane contribution). We use it to study the effect of boundary layer scale fluctuating pressure gradients ∂p′~/∂x, which are expected to be mostly two-dimensional, on the flow structure. Due to the imposed mean favorable pressure gradient (FPG) within the sink flow, the Reynolds stresses normalized by the local freestream velocity decrease over the entire boundary layer. However, when scaled by the inlet freestream velocity, the stresses increase close to the wall and decrease in the outer part of the boundary layer. This trend is caused by the confinement of the newly generated vortical structures in the near-wall region of the accelerating flow due to combined effects of downward mean flow, and stretching by velocity gradients. Within both the zero pressure gradient (ZPG) and FPG boundary layers, sweeping motions mostly occur during positive fluctuating pressure gradients ∂p′~/∂x>0 as the fluid moving towards the wall is decelerated by the presence of the wall. Vorticity is depleted in the near-wall region, as the wall absorbs −ω′ from the flow by viscous diffusion. On the other hand, ejections occur mostly during periods of favorable fluctuating pressure gradients ∂p′~/∂x<0. During these periods, there is more viscous flux of vorticity −ω′ into the flow, since ∂−ω′/∂y<0 at the wall. Large scale ejection motions associated with ∂p′~/∂x<0 are more likely to transport smaller scale turbulence to the outer region of the boundary layer, while turbulence remains largely confined close to the wall due to the sweeping motions accompanying ∂p′~/∂x>0. During periods of ∂p′~/∂x>0 in the ZPG boundary layer, sweeps tend to increase the momentum in the near-wall region, whereas the adverse pressure gradient decelerates the fluid. These competing effects result in an unstable ω′<0 shear layer which rolls up into coherent vortical structures and increases ω′ω′ near the wall as compared to periods of ∂p′~/∂x<0. Due to the strong mean acceleration of the flow and weaker sweeps in the FPG boundary layer, the formation of an unstable shear layer, and hence vortical structures, is suppressed, decreasing the enstrophy close to the wall as compared to periods of ∂p′~/∂x<0.

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Experimental Study on the Effect of Vortex Generator on the Aerodynamic Characteristics of NASA LS-0417 Airfoil

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.758.63 ◽

2015 ◽

Vol 758 ◽

pp. 63-69 ◽

Cited By ~ 1

Author(s):

S. Sutardi ◽

Agung E. Nurcahya

Keyword(s):

Boundary Layer ◽

Experimental Study ◽

Wind Tunnel ◽

Vortex Generator ◽

Boundary Layer Separation ◽

Chord Length ◽

Vortex Generators ◽

Airfoil Surface ◽

Freestream Velocity ◽

Drag And Lift

Boundary layer flow structure developing on an airfoil surfaces strongly affects drag and lift forces acting on the body. Many studies have been done to reduce drag, such as introducing surface roughness on the airfoil surface, gas injection, attachment of vortex generators, or moving surface on the airfoil. Previous results showed that the attachment of vortex generators has potentially been able to control boundary layer separation compared to other controlling devices. This study is focused on the evaluation of the effect of vortex generator attachment on the NASA LS-0417 airfoil profile as this profile is commonly used in wind turbine blade application. The models of this experimental study are NASA LS-0417 profiles, with and without vortex generator. The chord length of the profile is 110 mm, while the span is 210 mm. Profile of the vortex generator is a symmetrical profile of NACA 0012 configured in counter rotating and attached on the upper surface of the main profile. The chord length of the vortex generator is 7 mm with two different values of the height (h): 1 mm and 2 mm. The experiment was conducted in an open loop wind tunnel with maximum attainable freestream velocity of approximately 19 m/s and the turbulence intensity at the tunnel centerline is approximately 0.8%. The wind tunnel cross section is octagonal of 30 cm x 30 cm and of 45 cm to 60 cm adjustable length. The study was performed at two different freestream velocities of 12 m/s and 17 m/s corresponding with Reynolds numbers (Re) of 0.83 x 105 and 1.18 x 105 based on the airfoil chord length and the freestream velocity. Angle of attact (α) was varied from 0o to 24o. Drag and lift were measured using a force balance with measurement uncertainty of approximately 0.77% and 2.47% at measured drag of 0.65N and at measured lift of 0.202N, respectively. A flow visualization study using oil flow method was conducted to obtain qualitaive picture of flow structure on the airfoil surface. Results of this study showed that attachment of the vortex generator on the NASA LS-0417 profile has not been able to improve the profile performance compared to that of unmodified profile. There, however, seems Reynolds number effect on the airfoil performance flow conditions performed in this study. At lager Re, there is an increase in CL/CD of approximately 36% at angle of attack (α) 6o. Next, based on the flow visualization results, attachment of the 2mm vortex generator on the airfoil NASA LS-0417 surface results in an advancement of boundary layer separation at the two Re’s conducted in this study. Finally, the 2mm vortex generator accelerates airfoil stall at approximately 16o, while the 1mm vortex generator is relatively no effect on the airfoil stall angle.

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Turbulence mechanism in Klebanoff transition: a quantitative comparison of experiment and direct numerical simulation

Journal of Fluid Mechanics ◽

10.1017/s0022112002007954 ◽

2002 ◽

Vol 459 ◽

pp. 217-243 ◽

Cited By ~ 113

Author(s):

S. BAKE ◽

D. G. W. MEYER ◽

U. RIST

Keyword(s):

Numerical Simulation ◽

Boundary Layer ◽

Direct Numerical Simulation ◽

Outer Part ◽

Numerical Data ◽

Transition Process ◽

Shear Layers ◽

High Shear ◽

Vortical Structures ◽

Randomization Process

The mechanism of turbulence development in periodic Klebanoff transition in a boundary layer has been studied experimentally and in a direct numerical simulation (DNS) with controlled disturbance excitation. In order to compare the results quantitatively, the flow parameters were matched in both methods, thus providing complementary data with which the origin of turbulence in the transition process could be explained. Good agreement was found for the development of the amplitude and shape of typical disturbance structures, the Λ-vortices, including the development of ring-like vortices and spikes in the time traces. The origin and the spatial development of random velocity perturbations were measured in the experiment, and are shown together with the evolution of local high-shear layers. Since the DNS is capable of providing the complete velocity and vorticity fields, further conclusions are drawn based on the numerical data. The mechanisms involved in the flow randomization process are presented in detail. It is shown how the random perturbations which initially develop at the spike-positions in the outer part of the boundary layer influence the flow randomization process close to the wall. As an additional effect, the interaction of vortical structures and high-shear layers of different disturbance periods was found to be responsible for accelerating the transition to a fully developed turbulent flow. These interactions lead to a rapid intensification of a high-shear layer very close to the wall that quickly breaks down because of the modulation it experiences through interactions with vortex structures from the outer part of the boundary layer. The final breakdown process will be shown to be dominated by locally appearing vortical structures and shear layers.

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Direct Numerical Simulation of Flow Past a Sphere in a Plane Turbulent Boundary Layer with Immersed Boundary Method

Proceedings of the 2nd International Conference on Simulation and Modeling Methodologies, Technologies and Applications ◽

10.5220/0004056802430249 ◽

2012 ◽

Keyword(s):

Numerical Simulation ◽

Boundary Layer ◽

Turbulent Boundary Layer ◽

Direct Numerical Simulation ◽

Immersed Boundary Method ◽

Immersed Boundary ◽

Flow Past ◽

Boundary Method ◽

Flow Past A Sphere

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Numerical simulation of the effect of a moving wall on separation of flow past a symmetrical aerofoil

Proceedings of the Institution of Mechanical Engineers Part A Journal of Power and Energy ◽

10.1243/0957650981536736 ◽

1998 ◽

Vol 212 (1) ◽

pp. 69-77 ◽

Cited By ~ 6

Author(s):

Y T Chew ◽

L S Pan ◽

T S Lee

Keyword(s):

Numerical Simulation ◽

Reynolds Number ◽

Lift Coefficient ◽

Leading Edge ◽

Surface Velocity ◽

Freestream Velocity ◽

Moving Wall ◽

Rotating Circular Cylinder ◽

Flow Past ◽

Numerical Simulation Technique

This paper applies the numerical simulation technique based on the generalized conservation of circulation (GCC) method to investigate the effects of a leading-edge rotating circular cylinder on the suppression of stall flow past a symmetrical Joukowski aerofoil. The variables investigated were the angle of attack α and the ratio of the surface velocity of the cylinder to freestream velocity, CU. The Reynolds number based on chord length is 1.43 × 105. It was found that the separation point on the upper surface of the aerofoil shifts downstream with increasing CU and stall flow can be significantly suppressed even at α up to 30° when CU = 4. The lift coefficient CL increases and the drag coefficient Cd decreases with increasing CU and the optimum CL/ Cd occurs at α=80°. The maximum CL/ Cd obtained is about 60 at CU = 4.

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