Mean Base Pressure Research Articles

Instantaneous Structure of the Near-Wake of a Circular Cylinder: on the Effect of Reynolds Number

The near-wake structure of a cylinder is examined for Reynolds numbers from 103 to 104, with the intent of providing insight into the large changes in mean base pressure and fluctuating lift observed in previous investigations. The use of high-image-density particle image velocimetry allows the generation of instantaneous and averaged representations of velocity fields, streamline topology, and vorticity distribution. Modes of large-scale (Kármán) vortex formation, in conjunction with patterns of small-scale (Kelvin-Helmoltz) vortical structures, allow physical interpretation of the vortex formation length, which is the traditional criterion for linking the flow pattern to the loading coefficients.

Timing of vortex formation from an oscillating cylinder

The instantaneous structure of the near-wake of a cylinder subjected to forced oscillations is examined using particle imaging, which leads to representations of the streamline patterns and distributions of vorticity. As the frequency of excitation of the cylinder is increased relative to the inherent vortex formation frequency, the initially formed concentration of vorticity moves closer to the cylinder until a limiting position is reached; at this position, the vorticity concentration abruptly switches to the opposite side of the cylinder. This process induces abrupt changes of the topology of the corresponding streamline patterns; such topological patterns alone, however, do not properly suggest the existence and rearrangement of the vorticity concentrations. Moreover, this vorticity-switching concept persists to high values of Reynolds number, where the values of the mean base pressure coefficient and vortex formation length differ substantially from those at low Reynolds number. The switching mechanism is not significantly altered, either in an instantaneous or ensemble-averaged sense, by the presence of small-scale Kelvin–Helmholtz vortices that coexist with the large-scale (Kármán) vortices.

Base pressure coefficients for flows around rectangular plates

The flow around long rectangular plates can be modified by acoustic forcing. The leading-edge vortex shedding is locked to the sound frequency over a broad range of flow velocities. Close to a critical acoustic Strouhal number, significant drops in the mean base pressure coefficient may occur, depending on the plate chord. It is hypothesised that the plate chord changes the position in the acoustic phase at which the leading-edge vortices arrive at the trailing edge. Interference with the trailing-edge vortex shedding may then be responsible for the significant variation in mean base pressure coefficient. This situation is not observed for flows around plates with no leading-edge shedding.

Turbulence and blockage effects on two dimensional rectangular cylinders

This paper presents a review of some measurements of turbulence scale effect on the mean drags of rectangular cylinders. In the case of square cylinders exposed to a fixed turbulence intensity, two sets of data indicate a turbulence scale effect on the mean base pressure (∼ 25% reduction) for a turbulence scale twenty fold the cylinder frontal dimension. For smaller scales, the square cylinders mean drag appears influenced principally by the turbulence intensity. This paper also reviews some of the measurements of solid blockage effects on the mean drags and base pressures of two dimensional rectangular cylinders. The different methods are usually empirical and based on slopes of data that differ from one wind tunnel to another. An empirical method taking into account the models aspect ratio is proposed. Its application to smooth flow data (6 different sets) has given good corrected results. The method has been applied to data of models exposed to a turbulent flow and flows with incidence.

On vortex formation from a cylinder. Part 2. Control by splitter-plate interference

Control of vortex formation from a circular cylinder by a long plate in its wake is examined over the Reynolds number range 140 < Re < 3600. There are two basic flow regimes: a pre-vortex formation regime, in which the plate precludes formation of a large-scale vortex upstream of the tip of the plate; and a post-vortex formation regime in which one or more large-scale vortices are formed upstream of the edge. The unsteady pressure loading at the tip of the plate increases by over an order of magnitude during transition from the pre- to post-vortex formation regime. If the plate is located near the cylinder, it is possible to more than double the vortex formation length, relative to the case of the free wake. Moreover, these observations suggest that: there is a minimum streamwise lengthscale for development of the absolute instability of the near wake and thereby the large-scale vortex; and the vortex formation length may also be influenced by the downstream vorticity dynamics. When the plate is located downstream of the initially formed vortex, effective control is possible when the near-wake fluctuation level and mean base pressure of the corresponding free (non-impinging) wake are sufficiently small. This occurs in the low and moderate subcritical regimes; the substantial control by the wake-plate interaction in this range of Reynolds number implies low strength of the absolute instability of the near wake. However, in the pure von Kármán regime, selfcontrol of the near wake dominates that imposed by the wake-edge interaction, suggesting a strong absolute instability of the near wake.

On vortex formation from a cylinder. Part 1. The initial instability

Vortex shedding from a circular cylinder is examined over a tenfold range of Reynolds number, 440 ≤ Re ≤ 5040. The shear layer separating from the cylinder shows, to varying degrees, an exponential variation of fluctuating kinetic energy with distance downstream of the cylinder. The characteristics of this unsteady shear layer are interpreted within the context of an absolute instability of the near wake. At the trailing-end of the cylinder, the fluctuation amplitude of the instability correlates well with previously measured values of mean base pressure. Moreover, this amplitude follows the visualized vortex formation length as Reynolds number varies. There is a drastic decrease in this near-wake fluctuation amplitude in the lower range of Reynolds number and a rapid increase at higher Reynolds number. These trends are addressed relative to the present, as well as previous, observations.

Vortex excitation of rectangular cylinders with a long side normal to the flow

The vortex excitation of rectangular cylinders having side ratios of 0.2, 0.4 and 0.6, with a long side normal to the flow, in a mode of lateral translation, is investigated experimentally in a wind tunnel using free- and forced-oscillation methods. The range of reduced wind speeds investigated, 3-12, includes the vortex-resonance regime. The forced-oscillation experiment includes measurements of the fluctuating lift force at amplitudes up to 10% of the length of a long side. The experiments were also performed on cylinders with a long fixed splitter plate. The results of the measurements show that vortex excitation of a rectangular cylinder is strongly dependent on the side ratio. It is suggested that the critical change of the mean base pressure of an oscillating rectangular cylinder with increasing side ratio is closely correlated with the vortex-excitation characteristics. The concept of vortex excitation as aeroelastic flutter occurring in a fluid-body coupled system is proposed on the basis of the experimental results. The most essential feature of vortex excitation as a coupled flutter is that the fluid subsystem can be set into resonance by the body motion. A rapid phase-angle change in the lift force through vortex resonance produces a large out-of-phase force component which excites the motion of the body subsystem.

Rectangular Cylinders in Flows with Harmonic Perturbations

The effects of small amplitude harmonic perturbations on the pressure coefficients and wake development behind rectangular cylinders are examined. Two body configurations with slenderness ratios (streamwise body dimension/transverse body dimension) of B/D=0.5 and 2.0 are used. For the B/D=0.5 body, with the pulsation frequency equal to twice or four times the natural shedding frequency, and with amplitudes of the order of 1% of the freestream velocity, the mean base pressure shows a decrease of up to 18% when compared with its value in steady flow. Flow visualization shows this decrease in mean base pressure to be associated with increased vortex strength, decreased vortex formation length, and the corresponding increase in shear layer curvature. When the perturbation amplitude exceeds an undetermined threshold value at a frequency corresponding to four times the natural shedding frequency, two vortices are shed simultaneously and symmetrically with respect to the body centerline. For the B/D=2.0 body, the primary effect of the pulsations is to enhance the orderly structure of the separated shear layers.

The effect of a perturbation on the flow over a bluff cylinder

Measurements are presented of the frequency of vortex shedding from, in turn, a circular, a D-section cylinder, and a flat plate for a flow that has a single-frequency perturbation superimposed on its mean velocity component. It is found that over a range of velocities the vortex shedding locks-in to the frequency of the perturbation such that the shedding frequency is half the perturbation frequency. Also, the range of velocities for which lock-in occurs increases as the perturbation amplitude is increased. With lock-in, the mean base pressure is found to decrease, and measurements of the vortex shedding from the circular cylinder show that the correlation of vortex shedding increases. The changes associated with lock-in are found to be greatest for the circular cylinder.

Some Effects of Upstream Turbulence on the Unsteady Lift Forces Imposed on Prismatic Two Dimensional Bodies

Measurements of fluctuating lift are reported from four fixed rectangular prisms and a fixed circular cylinder, for various intensities of turbulence in the oncoming stream. Variations of unsteady lift with intensity of small scale turbulence are similar to present (and previous) measured variations of mean drag or base pressure. Reasons are suggested for these trends. Observations are also reported of the maximum (resonant) RMS amplitude of two cylinders, one square and the other circular in cross section, free to move in the cross stream direction. Small scale free stream turbulence provided by the wake of a small upstream rod increased the resonant amplitude of motion in both cases, the greater increase (and subsequent decrease) occurring for the square section. Two possible explanations for these observations are advanced.

The effects of turbulence on the mean flow past two-dimensional rectangular cylinders

There are two main effects of turbulence on the mean flow past rectangular cylinders, just as found earlier for square rods. Small-scale turbulence increases the growth rate of the separated shear layers through increased mixing. Large-scale turbulence weakens regular vortex shedding by reducing spanwise correlation. The consequences of increased mixing and weakened regular vortex shedding depend on the depth-to-height ratio of a rectangular cylinder. In particular, the mean base pressure of a rectangular cylinder can be varied significantly by both the scale and intensity of turbulence.

The effects of turbulence on the mean flow past square rods

There are two main effects of turbulence on the mean flow past rods of square cross-section aligned with the approaching flow. Small-scale turbulence increases the growth rate of the shear layer, while large-scale turbulence enhances the roll-up of the shear layer. The consequences of these depend on the length of a square rod. The mean base pressure of a square rod varies considerably with turbulence intensity and scale as well as with its length.

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

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.

The effects of turbulence intensity and scale on the mean flow past square rods

Measurements on square rods with their square face normal to turbulent flow show that the mean base pressure can be varied considerably with turbulent intensity and scale as well as with the thickness of a rod. Turbulence decreases the mean base pressure of a short rod while it increases the mean base pressure of a long rod. Turbulence causes earlier reattachment of the separated shear layers to the sides of a long rod. It is suggested that the effect of turbulence on the mean base pressure of a rod is restricted to a certain finite range of turbulence scale. There is a subrange of turbulence scale where the mean base pressure, although very different from the smooth-flow value, remains almost unchanged with turbulence scale.

A Method for Reducing the Base Drag of Wings with Blunt Trailing Edge

SummaryTo study the possibilities of reducing the base drag of profiles with a blunt trailing edge, experiments were performed in two low-speed wind-tunnels at the DFVLR-AVA, Göttingen, some on models between walls and some on rectangular wings with an aspect ratio of 2.5. The results show that the mean base pressure can be increased, and so the base drag reduced, by using a special form of the blunt trailing edge. The variation of local base pressure along the span, and the way in which this variation is influenced by the form of the trailing edge, is also shown. Some results for the total drag and lift are also indicated.

An investigation of the forces on flat plates normal to a turbulent flow

Measurements on square and circular plates in turbulent flow show the mean base pressure to be considerably lower than that measured in smooth flow. Power spectral density measurements of the fluctuating component of the drag on square plates in both smooth and turbulent flow are presented. The measurements in turbulent flow show the importance of the ratio of turbulence scale to plate size. There is shown to be a strong correlation between the fluctuating drag force and the velocity fluctuations in the approaching flow. The distortion of the turbulence structure approaching a plate is also discussed.