Vortex Shedding in Oscillatory Flow: Effects of Free-Stream Turbulence

1993 ◽  
pp. 123-126 ◽  
Author(s):  
Khalfan Al-Asmi ◽  
Ian P. Castro
Keyword(s):  
Vortex Shedding ◽  
Oscillatory Flow ◽  
Free Stream ◽  
Free Stream Turbulence ◽  
Flow Effects

An Experimental Investigation of the Wake of an Axisymmetric Body with a Slanted Base

1983 ◽  
Vol 34 (1) ◽  
pp. 24-45 ◽  
Author(s):  
X.J. Xia ◽  
P.W. Bearman
Keyword(s):  
Drag Coefficient ◽  
Vortex Shedding ◽  
Free Stream ◽  
Sudden Change ◽  
Base Pressure ◽  
Wake Flow ◽  
Main Body ◽  
Angle Of Incidence ◽  
Slant Angle ◽  
Free Stream Turbulence

SummaryThe effect of base slant on the base pressure distribution, drag coefficient and vortex shedding characteristics of a model consisting of an axisymmetric main body with an ellipsoidal nose have been investigated for three fineness ratios; 3, 6 and 9. A sudden change in the drag coefficient and separated flow pattern is observed at a critical slant angle (for constant incidence) or at a critical angle of incidence (for a constant base slant angle). The tests confirm that the value of the maximum drag coefficient is extremely sensitive to angle of incidence. Measurements of the frequency of vortex shedding are presented and the structure of the wake is investigated using smoke visualization and hot-wire correlation measurements. The wake is found to be far less stable than that from a two-dimensional bluff body and the vortex structures are sometimes in-phase and sometimes out of phase across the wake. The effect of free-stream turbulence on this family of body shapes is observed to be different to that on three-dimensional blunt-faced bluff bodies. Free-stream turbulence is found to have a minimal effect on base pressure for slant angles giving a recirculating type near wake flow. When longitudinal vortices are present the addition of free-stream turbulence slightly reduces the magnitude of the peak suctions recorded on the base but has little effect on base drag.

Effect of Elevated Free-Stream Turbulence on Transitional Heat Transfer Over Dual-Scaled Rough Surfaces

2003 ◽  
Author(s):  
Ting Wang ◽  
Matthew C. Rice
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Surface Roughness ◽  
Vortex Shedding ◽  
Free Stream ◽  
Leading Edge ◽  
Test Surface ◽  
Dominant Effect ◽  
Free Stream Turbulence ◽  
Heat Transfer Phenomenon

The surface roughness over a serviced turbine airfoil is usually multi-scaled with varying features that are difficult to be universally characterized. However, it was previously discovered in low freestream turbulence conditions that the height of larger roughness produces separation and vortex shedding, which trigger early transition and exert a dominant effect on flow pattern and heat transfer. The geometry of the roughness and smaller roughness scales played secondary roles. This paper extends the previous study to elevated turbulence conditions with free-stream turbulence intensity ranging from 0.2–6.0 percent. A simplified test condition on a flat plate is conducted with two discrete regions having different surface roughness. The leading edge roughness is comprised of a sandpaper strip or a single cylinder. The downstream surface is either smooth or covered with sandpaper of grit sizes ranging from 100 ∼ 40 (Ra = 37 ∼ 119 μm). Hot wire measurements are conducted in the boundary layer to study the flow structure. The results of this study verify that the height of the largest-scale roughness triggers an earlier transition even under elevated turbulence conditions and exerts a more dominant effect on flow and heat transfer than does the geometry of the roughness. Heat transfer enhancements of about 30 ∼ 40 percent over the entire test surface are observed. The vortical motion, generated by the backward facing step at the joint of two roughness regions, is believed to significantly increase momentum transport across the boundary layer and bring the elevated turbulence from the freestream towards the wall. No such long-lasting heat transfer phenomenon is observed in low FSTI cases even though vortex shedding also exists in the low turbulence cases. The heat transfer enhancement decreases, instead of increases, as the downstream roughness height increases.

Effect of Free-Stream Turbulence on Characteristics of Fluctuating Forces Acting on Two Square Prisms in Tandem Arrangement

1988 ◽  
Vol 110 (2) ◽  
pp. 140-146 ◽  
Author(s):  
H. Sakamoto ◽  
H. Haniu
Keyword(s):  
Reynolds Number ◽  
Strouhal Number ◽  
Turbulence Intensity ◽  
Vortex Shedding ◽  
Free Stream ◽  
Switching Frequency ◽  
Tandem Arrangement ◽  
Free Stream Turbulence ◽  
Fluid Forces ◽  
Bistable Flow

The effect of the addition of the turbulence intensity to the free stream on the characteristics of the bistable flow which takes place around two square prisms in tandem arrangement was studied experimentally at a Reynolds number of 3.32 × 104. A method of obtaining the fluid forces acting on two prisms in the bistable flow regimes where two flow patterns appear intermittently was introduced, and then the characteristics of the fluid forces, the Strouhal number, and the switching frequency of the switch phenomenon with the variation of the freestream turbulence intensity were investigated. Furthermore, the behavior of the fluid forces and the vortex shedding for other spacings between the two prisms were presented for the variation of the turbulence intensity.

On the dynamics of three-dimensional slung prisms under very low and high turbulence flows

2017 ◽  
Vol 816 ◽  
pp. 468-480 ◽  
Author(s):  
Y. Jin ◽  
L. P. Chamorro
Keyword(s):  
Growth Rate ◽  
Vortex Shedding ◽  
Small Amplitude ◽  
Free Stream ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Free Stream Turbulence ◽  
Aspect Ratios ◽  
High Turbulence ◽  
Hotwire Anemometry

The distinctive pendulum-like oscillation and pitching patterns of cubic and rectangular slung prisms were inspected for two aspect ratios at various Reynolds numbers $Re$ under two free-stream turbulence levels. Systematic experiments were performed using high-resolution telemetry and hotwire anemometry to quantitatively characterize the dynamics of the prisms and the wake fluctuation. The results show that the dynamics of the prisms can be characterized by two distinctive regions depending on the prism shape. Specifically, in the case of cubic prisms the regions are defined by the growth rate of the pitching amplitude; whereas the dynamics of the rectangular prisms is more sensitive to the angle of attack. In particular, when the large side initially faces the flow, the regions are defined by the synchronization between the vortex shedding and pure oscillations under very low turbulence. When the smaller side initially faces the flow, the regions are defined by the equilibrium pitching position. Regardless of the geometry of the prism and flow condition the dominant oscillation frequency resulted as being close to the natural frequency of the small-amplitude pendulum-like oscillation.

The effect of free-stream turbulence on sectional lift forces on a circular cylinder

1996 ◽  
Vol 306 ◽  
pp. 267-292 ◽  
Author(s):  
H. M. Blackburn ◽  
W. H. Melbourne
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Free Stream ◽  
Reynolds Numbers ◽  
Wind Tunnel Experiments ◽  
Number Range ◽  
Correlation Lengths ◽  
Free Stream Turbulence ◽  
Smooth Flow ◽  
Lift Forces

Wind-tunnel experiments were conducted to examine the effect of grid-generated turbulence on lift forces at sections of a circular cylinder. Turbulence of longitudinal intensity between 0.6% and 18% was employed, with cylinder Reynolds numbers in the range 1 × 105 to 5 × 105. Addition of low-intensity turbulence had the primary effect of inducing the critical transition at Reynolds numbers below that for smooth flow; above transition there was little difference between the forces experienced by the cylinder in smooth or turbulent flow, with no sign of organized vortex shedding.At higher turbulence intensities effects consistent with a return to organized vortex shedding were observed, particularly for the highest intensity and at the upper end of the Reynolds number range; lift coefficients were greater than in smooth supercritical flow, with a broad spectral peak centred near a Strouhal number of 0.23 accompanied by an increase in spanwise correlation lengths of lift force.

Effect of Elevated Free-Stream Turbulence on Transitional Flow Heat Transfer Over Dual-Scaled Rough Surfaces

2005 ◽  
Vol 127 (4) ◽  
pp. 393-403 ◽  
Author(s):  
Ting Wang ◽  
Matthew C. Rice
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Surface Roughness ◽  
Vortex Shedding ◽  
Free Stream ◽  
Leading Edge ◽  
Transitional Flow ◽  
Test Surface ◽  
Dominant Effect ◽  
Free Stream Turbulence

The surface roughness over a serviced turbine airfoil is usually multiscaled with varying features that are difficult to be universally characterized. However, it was previously discovered in low free-stream turbulence conditions that the height of larger roughness produces separation and vortex shedding, which trigger early transition and exert a dominant effect on flow pattern and heat transfer. The geometry of the roughness and smaller roughness scales played secondary roles. This paper extends the previous study to elevated turbulence conditions with free-stream turbulence intensity ranging from 0.2% to 6.0%. A simplified test condition on a flat plate is conducted with two discrete regions having different surface roughness. The leading-edge roughness is comprised of a sandpaper strip or a single cylinder. The downstream surface is either smooth or covered with sandpaper of grit sizes ranging from 100 to 40 Ra=37-119 μm. Hot wire measurements are conducted in the boundary layer to study the flow structure. The results of this study verify that the height of the largest-scale roughness triggers an earlier transition even under elevated turbulence conditions and exerts a more dominant effect on flow and heat transfer than does the geometry of the roughness. Heat transfer enhancements of about 30–40%-over the entire test surface are observed. The vortical motion, generated by the backward facing step at the joint of two roughness regions, is believed to significantly increase momentum transport across the boundary layer and bring the elevated turbulence from the freestream towards the wall. No such long-lasting heat transfer phenomenon is observed in low free-stream turbulence cases even though vortex shedding also exists in the low turbulence cases. The heat transfer enhancement decreases, instead of increases, as the downstream roughness height increases.

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