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.

Experiments on the flow past an inclined disk

1967 ◽  
Vol 29 (4) ◽  
pp. 691-703 ◽  
Author(s):  
J. R. Calvert
Keyword(s):  
Vortex Shedding ◽  
Free Stream ◽  
Stream Velocity ◽  
Length Scale ◽  
Angle Of Incidence ◽  
Axially Symmetric ◽  
Periodic Motions ◽  
Reference Velocity ◽  
Shedding Frequency ◽  
A Chain

The wake of a disk at an angle to a stream contains marked periodic motions which arise from the regular shedding of vortices from the trailing edge. The vortices are in the form of a chain of irregular rings, each one linked to the succeeding one, and they move downstream at about 0·6 of free-stream velocity. The prominence of the vortex shedding increases as the angle of incidence (measured from the normal) increases up to at least 50°. The shedding frequency increases with the angle of incidence, but by a suitable choice of reference velocity and length scale, may be described by a wake Strouhal number which has the constant value 0·21 for all angles of incidence above zero, up to at least 40°.Axially-symmetric bodies at zero incidence shed vortices in a similar manner, except that the orientation of the plane of vortex shedding is not fixed and varies from time to time.

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.

An Investigation of the Effects of Incidence on the Flow Around a Square Section Cylinder

1983 ◽  
Vol 34 (4) ◽  
pp. 243-259 ◽  
Author(s):  
E.D. Obasaju
Keyword(s):  
Vortex Shedding ◽  
Pressure Coefficient ◽  
Rate Of Change ◽  
Base Pressure ◽  
Angle Of Incidence ◽  
Spacing Ratio ◽  
Spectral Peaks ◽  
Vortex Shedding Frequency ◽  
The Mean ◽  
Shedding Frequency

SummaryA study has been made of the changes that take place in the flow around a square section cylinder as the angle of incidence is increased from 0° to 45°. Measurements of the Strouhal number, S, and the vortex longitudinal spacing, a/d, are presented and used to estimate the vortex strength,, and vortex street spacing ratio, b/a.is found to vary between about 1.2 and 1.7 depending on incidence, and is given approximately by 0.52(1 - Cpb)/2πS, where Cpbis the mean base pressure coefficient. As the incidence is increased from 0°, S at first decreases slightly and then rises sharply to a maximum at 13.5° incidence, which is the incidence where reattachment of the shear layer, in some mean sense, is expected to commence. The spectra of pressure and velocity fluctuations were measured and subharmonic peaks were found in both spectra at 5° and 10° incidence. It is suggested that they may have been caused by an interaction between a vortex and a trailing edge corner. The degree of organisation of the vortex shedding process was estimated by calculating the sharpness factor, Q, of the spectral peaks at the vortex shedding frequency. In general Q fluctuated with changes in incidence. High values of Q occurred at angles of incidence where the rate of change of the mean base pressure coefficient with incidence is very small whereas low values occurred where the flow is changing to a different state.

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.

Effect of Free Stream Turbulence on the Drag Coefficient of Bluff Sharp-edged Cylinders

Nature ◽  
1969 ◽  
Vol 223 (5208) ◽  
pp. 828-829 ◽  
Author(s):  
F. G. MCLAREN ◽  
A. F. C. SHERRATT ◽  
A. S. MORTON
Keyword(s):  
Drag Coefficient ◽  
Free Stream ◽  
Free Stream Turbulence

Effect of Base Slant on Flow in the Near Wake of an Axisymmetric Cylinder

1980 ◽  
Vol 31 (2) ◽  
pp. 132-147 ◽  
Author(s):  
Thomas Morel
Keyword(s):  
Drag Coefficient ◽  
Three Dimensional ◽  
Local Maximum ◽  
The Body ◽  
Wake Flow ◽  
Force Measurements ◽  
Near Wake ◽  
Slant Angle ◽  
Cylinder Length ◽  
Axisymmetric Bodies

SummaryThe effects of slanting the base of a slender axisymmetric cylinder (length/diameter ratio of 9), aligned with the flow, was studied experimentally. The body was equipped with interchangeable rear ends covering a range of slant angles between 0° (vertical) and 70°. It was found that the base slant has a very dramatic effect on body drag, particularly in a relatively narrow range of slant angles where the drag coefficient exhibits a large local maximum (over-shoot). Detailed study of the flow showed that the drag overshoot is related to the existence of two very different Separation patterns on the slanted base. One pattern is similar to that found behind axisymmetric bodies with no base slant, and its main feature is the presence of a closed Separation region adjacent to the base. The other pattern is highly three-dimensional with two streamwise vortices forming along the sides of the slanted base. This pattern sets in very abruptly at a “critical” slant angle α ∼ 47°. Drag force measurements showed that, at first, the drag coefficient slowly increases with the slant angle, but then jumps suddenly upwards to more than double its baseline value (from CD = 0.24 to CD = 0.625) at the critical angle. At angles higher than that CD decreases again, and at 70° it is about equal to the baseline value. Further effects of the slant angle are the generation of a large side force and a significant increase in near-wake flow periodicity.

A Numerical Investigation of the Effects of Flow Pulsations on the Drag Force Over a Cylinder

2011 ◽  
Author(s):  
Eric D’herde ◽  
Laila Guessous
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Drag Coefficient ◽  
Natural Frequency ◽  
Vortex Shedding ◽  
Free Stream ◽  
Stream Velocity ◽  
Vortex Shedding Frequency ◽  
The Mean ◽  
Shedding Frequency

Flow over a cylinder is a fundamental fluid mechanics problem that involves a simple geometry, yet increasingly complex flow patterns as the Reynolds number is increased, most notably the development of a Karman vortex with a natural vortex shedding frequency fs when the Reynolds number exceeds a value of about 40. The goal of this ongoing study is to numerically investigate the effect of an incoming free-stream velocity pulsation with a mean Reynolds number of 100 on the drag force over and vorticity dynamics behind a circular cylinder. This paper reports on initial results involving unsteady, laminar and incompressible flows over a circular cylinder. Sinusoidal free-stream pulsations with amplitudes Av varying between 25% and 75% of the mean free-stream velocity and frequencies f varying between 0.25 and 5 times the natural shedding frequency were considered. Of particular interest to us is the interaction between the pulsating frequency and natural vortex shedding frequency and the resulting effects on drag. Interestingly, at frequencies close to the natural frequency, and to twice the natural frequency, a sudden drop in the mean value of the drag coefficient is observed. This drop in the drag coefficient is also accompanied by a change in the flow and vortex shedding patterns observed behind the cylinder.

Wake Flow Visualization of a Simplified Vehicle Model During Flow State Change

2019 ◽  
Author(s):  
Takuji Nakashima ◽  
Takashi Moriuchi ◽  
Yan Chao ◽  
Itsuhei Kohri
Keyword(s):  
Sudden Change ◽  
Stable State ◽  
Wake Flow ◽  
Dynamic Force ◽  
State Change ◽  
Flow State ◽  
Slant Angle ◽  
Fluid Force ◽  
Piv Measurement ◽  
Slant Surface

Abstract A flow around a three-dimensional bluff body such as an automobile sometimes exhibits a bi-stable state wherein two stable flow states exist for a single condition. Better aerodynamic characteristics can be obtained if we suppress or promote the flow state change between such bi-stable states. Hence, it is necessary to understand the trigger conditions and process of the flow state change. In this study, we investigated the transient aerodynamics of the Ahmed model with the slant angle of 32°, exceeding the critical angle of 30°, known to exhibit bi-stable state under crosswind conditions. Changing the Yaw angle by rotating the model, produced change in the flow state, accompanied by time delay. While continuously measuring fluid force, we performed PIV measurement triggered by a sudden change in fluid dynamic force corresponding to the flow state change. Using these methods, we realized the synchronous measurement of the fluid force and wake flow during the flow state change. At the beginning of the flow state change, flow velocity changed around the trailing edge of the slant surface. Subsequently, the separated flow above the slant surface increased. A gradual decrease of drag coefficient was observed before the flow state change though flow behavior associated with the drag change was not observed in the velocity field of PIV measurement.

Effect of Free Stream Turbulence on the Drag Coefficient of Bluff Sharp-edged Cylinders

Nature ◽  
1969 ◽  
Vol 224 (5222) ◽  
pp. 908-909 ◽  
Author(s):  
F. G. MCLAREN ◽  
A. F. C. SHERRATT ◽  
A. S. MORTON
Keyword(s):  
Drag Coefficient ◽  
Free Stream ◽  
Free Stream Turbulence
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