Vortex Shedding Frequency Research Articles

Strouhal number for boundary shear flow past a circular cylinder in the subcritical flow regime

Strouhal number (St) is an essential dimensionless quantity for characterizing the vortex-shedding frequency of the flow past a cylinder. In this study, the Strouhal number for a near-wall circular cylinder fully immersed in a boundary shear flow in the subcritical flow regime was experimentally investigated in a water flume. The velocity fluctuations and flow fields of the lee-wake were systematically measured with an Acoustic Doppler Velocimeter (ADV) and an upward-illumination Particle Image Velocimetry (PIV) system, respectively. Based on dimensional analyses, a non-dimensional shear parameter (K) is introduced to characterize the shear flow. An explicit expression between K and e/D is obtained on the basis of the boundary theory and verified with the flume measurements. Spectral analysis of wake velocities indicates that the multi-peaks can be identified as the cylinder gets closer to the wall. The PIV measurements indicate that the multi-peaks spectra are predominantly induced by the intense interactions between the lower shear layer and wall boundary layer. By fitting the experimental results, empirical relationships for the variations of St with K or e/D are finally established for the subcritical flow regime.

Influence of Heave Plate on the Flow-Induced Motions of a Floating Offshore Wind Turbine

Abstract Flow-induce motions (FIM) small-scale model tests were performed for the Jappaku floating offshore wind turbines (JPK), a FOWT developed to operate in Brazilian waters. This paper aims to investigate the presence of FIM on the JPK to show the importance of heave plate (HP) design and to show how HP mitigates FIM. Three different HP dimensions were tested and compared with the condition without HP. In addition, two different incidence angles of the current were tested, namely, 0 deg and 180 deg. The results showed amplitudes in the transverse direction similar to the diameter of the external platform column for the case without HP. These amplitudes are higher than the ones observed for previous deep-draft semi-submersibles found in the literature. Conversely, the largest HP dimensions implied in mitigation of the FIM amplitudes, i.e., very low amplitudes, were observed. The presence of the central column played an essential role in FIM and significantly modified the amplitudes in different current incidences. Due to the different diameters of the external and central columns, the FIM presented two different branches of response related to the vortex-shedding frequency around the columns. The results showed that significant FIM could occur for this specific JPK investigation even with HP. The HP design has a positive effect on reducing dynamic behaviors due to the wave and current incidences. Therefore, its design must be included in the preliminary stages of FOWT developments.

Effect of various structures on the shell-side flow over coil tube bundles

In response to the increasing number of applications and safety considerations for coil tube bundles, an investigation on the influence of structural parameters (the pitch diameter ratio between adjacent tube layers, a, the pitch diameter ratio in the same layer, b, and the helix angle, α) on the flow in the shell side over simplified three-layer bundles was carried out using detached-eddy simulation (DES), which was verified by previous studies. In this study, the local flow structures were discussed by observing a zoomed-in view of the gaps, which revealed that irregularly distributed vortices appeared. The spiral wound structure made the flow structures asymmetric and increased the lateral velocity between tubes of the same layer. By applying power spectral density (PSD) and continuous wavelet transform (CWT) functions, the time–frequency data of velocity fluctuations were analyzed to obtain the flow and evolution of coherent structures over time, revealing vortex shedding frequency bands and Strouhal numbers (St). In addition, multimodal Gaussian distribution approximations (MGDA) were employed to indicate that the shell-side flow was composed of multiple flow modes.

Flow-induced vibrations of a cylinder along a circular arc

An elastically mounted circular cylinder, immersed in a cross-current and free to move along a rectilinear path, is subjected to vortex-induced vibrations (VIV). These vibrations develop through a mechanism referred to as lock-in, where body motion and vortex shedding synchronize at a frequency that may deviate both from the oscillator natural frequency and from the vortex shedding frequency past a fixed cylinder. The present numerical study aims at extending the analysis to curved trajectories, by considering that the cylinder is free to translate along a circular path. The Reynolds number based on the body diameter ( $D$ ) and current velocity ( $U$ ) is set to $100$ . A wide range of path radii, from $0.05D$ to $10D$ , and values of the reduced velocity (inverse of the oscillator natural frequency non-dimensionalized by $D$ and $U$ ) up to $30$ are examined, for the concave and convex configurations, i.e. the circular path centre located upstream or downstream of the cylinder. Path curvature results in a major alteration of the flow–body system behaviour compared with rectilinear VIV, with substantially different evolutions in the concave and convex configurations. In addition to the typical lock-in mechanism, two subharmonic forms of synchronization, at half and one third of vortex formation frequency, are uncovered in the convex configuration. They coexist with a desynchronized regime where the body and the flow oscillate at incommensurable frequencies. The four interaction regimes exhibit contrasted trends in terms of structural response, spatiotemporal organization of the wake and associated forces. They particularly differ by their symmetry properties, which are closely linked to the possible reconfiguration of the oscillator due to mean fluid forcing.

Local Scour Patterns around a Bridge Pier with Cable-Wrapping

The performance of cable flow-altering bed scour countermeasures was experimentally evaluated based on the scour reduction, bed morphology, and the effects on the flow field. An unprotected 40 mm diameter pier was compared to piers protected with spiral cables (2, 4, 6, 8 and 10 mm diameters) wrapped at a 15-degree angle for two-bed sediment sizes with median grain sizes of 0.86 and 1.83 mm, for a cylinder Reynolds number of 7120. The scour depth was reduced by the cables by up to 52 percent compared to the unprotected pier case, a reduction that increased with increasing cable diameter for both sediment beds. Scour depth and sediment deposition varied by sediment size, where the scour hole was up to 45 percent deeper for the finer sediment bed than that of the coarser bed. Velocity and turbulence statistics showed that cables attenuated the flow within the scour hole by diminishing the downflow and horseshoe vortex, whereas in the case of finer sediment, spatially averaged turbulent kinetic energy and Reynolds shear stresses were respectively up to 1.4 and 1.8 times higher for the unprotected pier than the protected pier, resulting in scour depth reduction. The presence of the cable also reduced the vortex shedding frequency in the pier wake as indicated by a Strouhal number of around 0.175. The results demonstrate the potential of cable threading as a flow-altering scour countermeasure to reduce bridge pier scour.

Study on flow characteristics around the thermowell

In the paper, the effects of different Reynolds numbers on the flow and dynamic characteristics of the thermowell were studied. The results show that a “downwash” phenomenon occurs in the thermowell wake flow field, a “tip” vortex exists on the right, and a “mushroom” vortex lays at the bottom of the free end of the thermowell. The lengths of the reflux zone and the forms of vortex shedding are different on the different horizontal sections of a thermowell. With the increase of the Reynolds number, the size of the “mushroom” vortex gradually decreases, and the “tip vortex” gradually increases. At the same horizontal section, the length of the recirculation zone becomes shorter with the increase of the Reynolds number, and the vortex shedding form gradually changes from “2S” shape to the straight line shape. In the region below 50% height of the thermowell, with the increase of Reynolds number, the boundary layer separation line gradually moves forward, both flow resistance of the casing and “downwash” phenomenon gradually increase, and both lift force around the casing and vortex shedding frequency in the wake decrease. The flow resistance and vibration induced by vortex shedding are the leading causes of thermowell damage.

Wake transitions behind a streamwise rotating disk

Direct numerical simulations are performed to investigate the wake transitions of the flow normal to a circular rotating disk. The diameter-thickness aspect ratio of the disk is $\chi =50$ . The Reynolds number of the free stream is defined as $Re_s=U_\infty D/\nu$ , with incoming flow velocity $U_\infty$ , disk diameter $D$ , and kinematic viscosity of the fluid $\nu$ . The rotational motion of the disk is described by the Reynolds number of rotation $Re_r=\varOmega Re_s$ , with non-dimensional rotation rate $\varOmega =\frac {1}{2}\omega D/U_\infty$ , where $\omega$ is the angular rotation speed of the disk. Extensive numerical simulations are performed in the parameter space $50 \leqslant Re_s \leqslant 250$ and $0 \leqslant Re_r \leqslant 250$ , in which six flow regimes are identified as follows: the axisymmetric state, the low-speed steady rotation (LSR) state, the high-speed steady rotation (HSR) state, the low-speed unsteady rotation (LUR) state, the rotational vortex shedding state, and the chaotic state. Although plane symmetry exists in the wake when the disk is stationary, a small rotation will immediately destroy its symmetry. However, the vortex shedding frequencies and wake patterns of the stationary disk are inherited by the unsteady rotating cases at low $Re_r$ . A flow rotation rate jump is observed at $Re_s\approx 125$ . The LUR state is intermediate between the LSR and HSR states. Due to the rotational motion, the wake of the disk enters the steady rotation state earlier at large $Re_r$ , and is delayed into the vortex shedding state in the whole range of $Re_r$ . In the steady rotation states (LSR and HSR), the steady flow rotation rate is linearly correlated with the disk rotation rate. It is found that the rotation of the disk can restrain the vortex shedding. The chaotic state can be regularized by the medium rotation speed of the disk.

Three-dimensional numerical investigation on flow behaviors around a diversion dike

Three-dimensional numerical simulations of a flow past a non-submerged diversion dike are performed for various blockage ratios (1/6 ≤ β ≤ 1/3) and Froude numbers (0.152 ≤ Fr ≤ 0.253). The instantaneous and time-averaged flow characteristics, the evolution process of mixing layers, and the bed shear stress of the dike are analyzed. The blockage ratio β plays an important role in the momentum redistribution and turbulent kinetic energy of the diversion dike. As the blockage ratio increases, the flow velocity gradient and the vortex shedding frequency in the mixing layer significantly increase. The Froude number Fr has little effect on the flow field. A large recirculation zone is observed in the wake of the dike, and the recirculation length is more depended on the length of the dike rather than the Froude number. The bed shear stress around the dike increases significantly with the increase in the blockage ratio.

A study of scale effects in experiments of monopile scour protection stability

Small scale hydraulic experiments are widely used to model the stability of a scour protection around an offshore monopile foundation. Knowing the associated scale effects is important for evaluating the validity of the obtained experimental and design results. This paper provides a quantification analysis of scale effects that exist in monopile scour protection experiments, with a focus on the shear damage of a dynamically stable scour protection. Large scale models (scale ratio 1:8.33 and 1:16.67) and similar small scale models (scale ratio 1:50) have been adopted in the experiments with waves against current hydrodynamic conditions applied. For the waves and current conditions, the Froude scaling rule is used; for the armour stones, the so-called Best Model scaling rule suggested by Hughes (1993) is used. The scaling scheme achieves similarities between large and small scale models with regard to Shields number, relative density, geometry and settling velocity of particle. The scour protection damage patterns are measured and the three dimensional damage numbers are analysed. For better comparing the small and large scale test results, the small scale tests are performed repeatedly to obtain reliable damage results with associated range of deviation. Visual assessment of the damage patterns shows some agreements between small and large scale tests with regard to the damage and accretion locations. However, detailed analysis shows that the small scale tests introduce higher global and subarea damage numbers compared to large scale tests. The damage areas in small scale tests are larger than that in large scale tests. Significant lee-side damage due to the presence of lee-wake vortices is found in small scale tests. The dissimilarities of pile Reynolds number ( R e , D P ), ratio between Shields numbers ( θ m a x / θ c r ) and vortex shedding frequency in the different scaled models are believed to be the primary reasons of obtained scale effects. • Scale effects in monopile scour protection experiments are investigated quantitatively. • Large scale (1:8.33 and 1:16.67) and small scale (1:50) experiments are performed under wave opposing current conditions. • Repeated small scale experiments are carried out to obtain reliable mean damage numbers and 95% confidence intervals. • Scour protection damages are analyzed and compared between small and large scale experiments. • Time scale of scour protection progressive damage development is investigated. • Different Reynolds numbers ( R e , D P ) and ratios between Shields numbers ( θ m a x / θ c r ) are the main reasons for the scale effects.

Thermal fluid–structural interaction of three cylinders undergoing flow-induced vibration with cross thermal buoyancy

The effect of cross thermal buoyancy on the characteristic of flow-induced vibration and mixed convection of three circular cylinders is numerically studied. Two-dimensional simulations were conducted for a Reynolds number (Re) of 100 and five Richardson numbers (Ri) of 0–1.00. The range of the reduced velocity is 3 ≤ U* ≤ 15. Three circular cylinders C1, C2, and C3 are arranged in an equilateral triangle with C1 in upstream. The results show that the maximum amplitude of the C1 increases by up to 21% with considering cross thermal buoyancy in comparison to the case of Ri = 0. The galloping-like response is observed on the C1 at Ri = 1.00. The lift coefficient of three circular cylinders increases with the increase in Ri at U* ≥ 6. When U* exceeds a critical value, the vibrations of the C2 and C3 are in-phase, and the “2S” pattern is observed in the near-wake of three circular cylinders. The near-wake becomes wider, and the vortex shedding frequency increases at U* = 6 and Ri = 0.25 and 0.50. The higher the reduced velocity, the more significant is the effect of cross thermal buoyancy in enhancing heat transfer. The maximum space time-averaged Nusselt number increases by 10.42% in comparison to the case of fixed cylinders.

Mechanism of front stagnation point unsteadiness for flow around a square cylinder

The unsteady characteristics and mechanisms of the front stagnation point oscillation for the flow around a square cylinder are investigated with the two-dimensional time-resolved particle image velocimetry technique in the range of 1420 ≤ Re ≤ 8600. The front stagnation point oscillates with the frequency equal to the wake vortex-shedding frequency. The Reynolds number has no effect on the amplitude and the dimensionless frequency of the front stagnation point. It is revealed that the front stagnation point moves from the centerline to one side when the wake vortex from the opposite side is forming and moving toward the centerline. This growing wake vortex stays attached to the rear surface of the square cylinder. After the wake vortex is detached from the rear surface, the front stagnation moves from the maximum displacement back toward the centerline. Furthermore, the quantitative correspondence between the location of front stagnation point and the wake pattern is revealed via time-lag correlation. The movement of the front stagnation point and the variation of typical proper orthogonal decomposition coefficient of the wake are nearly opposite in phase.

Control of separated flow transition over a highly loaded compressor blade via dynamic surface deformation

The laminar separation and transition process seriously deteriorate the aerodynamic performance of a compressor blade at a low Reynolds number (Re). To suppress the laminar separation without reattachment over a highly loaded compressor blade at Re=3.5 × 105, an active flow control strategy based on dynamic surface deformation (DSD) is numerically investigated by large eddy simulations (LESs). The control mechanism of DSD is analyzed, and the vortex dynamics in the transition process are resolved. Furthermore, the loss mechanism is clarified based on the turbulent kinetic energy (TKE) budget analysis. For the uncontrolled case, the rolling-up and breakdown of large-scale hairpin vortices accelerate the generation of turbulent fluctuations, which distinctly extend the region of high-level TKE production term and thus cause large total pressure loss. As the DSD is activated at the oscillation frequency f0 (f0 is the vortex shedding frequency in separated shear layers), periodic pressure fluctuation waves are induced with the appearance of small-scale vortices. The convection of these vortices enlarges the local vorticity and makes the boundary layer velocity profile fuller, which successfully eliminates the large-scale laminar separation on the compressor blade. As such, the region of high-level TKE production term shrinks distinctly due to the weakened vortex dynamics, and the total pressure loss is reduced by 78.9%. With increasing the oscillation frequency from f0 to 2f0 or 4f0, the loss is slightly enlarged due to the rapid growth of turbulent boundary layers, but it is still lower than that of the uncontrolled case. The results demonstrate that DSD is feasible in controlling large-scale laminar separation, and there exists an optimal oscillation frequency.

Experimental verification and numerical simulation of a vortex flowmeter at low Reynolds numbers

This study presents experimental verification and numerical simulations of a vortex flow meter in the Reynolds number range between 8300 and 50,000. A custom-designed bluff body with a wedge back shape was used in the flowmeter. A shedding frequency of the flowmeter was measured in an air duct using a hot-film probe. To evaluate the accuracy of the flowmeter, a measurement uncertainty analysis was performed. Numerical simulations of the vortex flowmeter were performed with the open source code OpenFOAM. Transient simulations of periodic vortex shedding behind the bluff body were performed using different simulation methods depending on the pipe Reynolds number, such as Direct Numerical Simulation (DNS), Large Eddy Simulation (LES) and Unsteady Reynolds Averaged Navier Stokes (URANS) method. The simulated vortex shedding frequencies matched the experimental data very well. Experiments and simulations demonstrated a clear linear dependence of the shedding frequency on the volumetric flow rate over the entire range of Reynolds numbers. In addition, numerical simulations were used to study the main mechanisms of vortex formation and shedding behind the considered bluff body.

An LES study of the wake flow dynamics and heat transfer characteristics of two side-by-side finite wall-mounted square cylinders

This paper studies the wake flow dynamics and heat transfer characteristics of two side-by-side finite wall-mounted square cylinders through large-eddy simulations at Reynolds and Prandtl numbers of Re = 1.2 × 104 and Pr = 0.7. The influence of normalized center-to-center distances between cylinders (S/d, where d is the side of the cylindersʹ cross-section) is investigated for S/d = 2–5, while both cylinders pose an identical aspect ratio of AR = 7. First, the numerical approach is validated against the published experimental data. Then the instantaneous and time-mean wake flow structures, as well as the global quantities, turbulence statistics and heat transfer are studied. It is found that the biased/flip-flop flow at S/d = 2 turns into anti-phase coupled vortex shedding flow at S/d ≥ 3. This flow regime transition induces a relatively sharp change in drag coefficient and vortex shedding frequency, minimum values at S/d = 2 and maximum values at S/d = 3. The Nusselt number mildly changes at S/d ≥ 3. The turbulent deflected gap flow at S/d = 2 results in inequality of drag forces on and heat transfer from two cylinders. That is, the drag coefficient and Nusselt number are larger for the cylinder towards which the gap-flow is deflected. It is also shown that swinging the deflected gap flow at S/d = 2 is a more intense source of turbulence than the vortex shedding from lateral faces, albeit the Reynolds stress is magnified with increasing S/d. The flow interference is steadily reduced with S/d, almost faded at S/d = 5.

Fluid-structure interaction analysis on vibration characteristics of sleeve control valve

As a key device affecting the safety and economy of the pipeline system of nuclear power plants, the control valve regulates the medium pressure and flow rate. In this paper, taking the sleeve control valve as the research object, lift coefficient of flow field and vibration analysis of valve stem and plug are conducted with fluid–structure interaction (FSI) method to investigate the vibration characteristics of the sleeve control valve. The results show that the vortex moves downward and the intensity decreases with the rising of valve opening. The dominant frequency of vortex shedding occurs around 449 Hz. The lift coefficient CL is maximum at small valve opening. CL reaches about 2400 at Lmin-op, while CL is about 850 at Lmax-op. On M1 of stem, the total vibration level decreases with the rising of the valve opening, increasing by 4 % and 3 % on the Y-axis and Z-axis respectively. On M2 of plug, the total vibration level decreases with the decrease of the valve opening, and the total vibration level is down nearly 7.5 % on the X-axis. With the rising of the valve opening, the total vibration level difference between M1 and M2 is increasing, which reach 25.47 dB and 8.48 dB on the X-axis and Z-axis respectively. This work provides a certain reference value for the research on vibration reduction and noise reduction of sleeve control valves.

Experimental study on dynamic responses of a tensioned flexible vertical cylinder under waves and combined current and waves

The two degree-of-freedom (2DOF) dynamic responses of a flexible vertical cylinder under waves only and combined current and waves in a finite water depth were experimentally studied. The cylinder has multi-frequency response in both in-line (IL) and crossflow (CF) directions under waves. The response frequencies in the IL direction are multiples of wave frequency. The CF response frequencies include the multiples of wave frequency and the natural frequency of the cylinder, indicating that the CF response is controlled by both the wave actions and the vortex shedding. When the natural frequency of the cylinder is a multiple of the wave frequency, the CF vibration has the largest amplitude. A top tension can suppress the CF response more than the IL response amplitude, and the suppression effect increases with the increase of the top tension. With the decrease of the water depth, the response amplitudes in both directions decrease. Under combined current and waves, the IL response amplitude increases with the increase of KC number and the primary frequency of IL response is still the wave frequency. For a current velocity where VIV occurs, the superimposition of waves on current reduces the CF response amplitude because the vortex shedding frequency moves away from the natural frequency. The multi-frequencies of CF response are related to both the natural frequency of the cylinder and the wave frequency. The trajectories of the cylinder are mainly dependent on the ratio of the natural frequency to the wave frequency under waves or combined current and waves. There can be 2–6 cycles in the CF direction for each IL cycle with different wave frequencies and the top tension.

Study of the influence of current pulse parameters on kinematic and energy characteristics of a plasma actuator based on a moving electric arc

In this work, a parametric study of the flow induced during the movement of an arc channel in a transverse constant magnetic field in initially still air is carried out. The analysis of the flow structure, integral energy and kinematic characteristics of the arc acceleration process is carried out. Instantaneous velocity fields in the induced flow were experimentally obtained for the range of discharge current amplitudes from 8 to 160 A and pulse durations from 40 to 380 μs. A parametric study of the flow structure in the wake behind the arc is carried out by the method of two-dimensional numerical simulation in the MHD approximation. In the particular case of a single-mode current pulse, the shape of the current pulse is optimized to maximize the gas momentum in the near-wall region. It is shown that similar conditions for the balance of local acceleration, convective acceleration and viscous forces are observed for the problem of flow past an arc and a cylinder for regimes with a conditionally steady vortex shedding in the wake. In this case, the coincidence of the vortex shedding frequency occurs when the characteristic scale of the streamlined body is chosen equal to half the height of the thermal cavity.

Harvesting airflow energy from circular cylinder wake via a thin polyvinylidene fluoride film

To improve the characteristics of airflow energy harvesting, this paper presents a new piezoelectric wind energy harvester (PWEH), in which a polyvinylidene fluoride (PVDF) film was directly located in the wake of the circular cylinder. Four important parameters were independently varied for detailed experimental investigations: wind speed, spacing ratio for the distance between the center of the circular cylinder and the tip of the PVDF film to the circular cylinder diameter, diameter of the circular cylinder, and load resistance. The results indicated that the designed PWEH performed optimally with the occurrence of resonance, maintaining a relatively high output when significantly exceeding the resonance wind speed. The PVDF film inhibited the vortex shedding of the circular cylinder, which weakened with increasing spacing ratio. The optimum value of the spacing ratio should be approximately 2.5, which yields an effective coupling between the vortex and structure. The diameter of the circular cylinder played an important role in determining the output power. The match between the load resistance and PWEH also contributed to the high-level performance. Moreover, with an increase in the diameter of the circular cylinder, the increasing blockage ratio unexpectedly enhanced the vortex shedding frequency and decreased the theoretical resonance wind speed, which provided a new orientation for improving the performance of such PWEHs.

Flow-induced vibration of a circular cylinder with an attached elastic plate of high aspect ratio

The flow-induced transverse vibration of a cylinder (diameter D*) with an attached flexible and elastic plate of high aspect ratio to its leeward side is investigated numerically at a low Reynolds number of 150 for a range of reduced velocities (Ur) using an in-house developed fluid solver based on curvilinear immersed boundary method strongly coupled with an open-source finite element-based structural solver. It was observed that an attached elastic plate of width B=B*/D*=0.1 and length L=L*/D*=1 suppresses large vibrations of the cylinder, but one with length L = 2, contrary to previous studies, amplifies vibrations up to five times of an isolated cylinder. Three regimes were observed: vortex-induced vibration (VIV), suppression, and galloping. In VIV regime for 3≤Ur≤7, lock-in was observed where the vortex shedding frequency from the plate-cylinder system was seen to slightly increase relative to that of static cylinder–plate system to match with the natural frequency of the cylinder and the plate. In this regime, the deformations of the elastic plate were large (max. 91% of L) and in high modes (up to fifth mode), leading to new vortex patterns. The transverse displacement of the cylinder–plate system was found to reach nearly twice of an isolated cylinder in this regime. For 7<Ur≤9, the cylinder–plate system was pushed into suppression regime, wherein its displacement was nullified because of lack of vorticity interaction and out-of-phase deformation. Beyond Ur = 9, the cylinder–plate system vibrated in the galloping regime, wherein it shed and generated forces as an asymmetric body creating an angle of attack with the incoming flow. The primary mode of deformation of the elastic plate progressively increased from second mode to third mode in galloping regime, and the transverse displacement of cylinder showed a linear increase with the increase in reduced velocity until Ur = 18. The vibration amplitude of the cylinder was higher in the galloping regime, but the vibrations of the plate were more intense (higher amplitude and mode) in the VIV regime. New vortex patterns were observed in the VIV and galloping regimes ranging from 2S mode till 2T mode including all the vortex pattern between them like 2S, 2P, 2Q, and P + T modes, which are reported for the first time.

Visualization of the flow behind a backward-facing step using a schlieren technique and tetrafluoroethane

This paper explores the possibility of visualizing the low-speed, near-wall flow using a schlieren imaging technique. In this investigation, wind tunnel testing is conducted on a backward-facing step at step Reynolds numbers of 2940, 5140, and 6120. Tetrafluoroethane is injected through a rectangular slot mounted flush to the wind tunnel floor and is introduced to the downstream region of the step as refractive tracers. The visualization acquired at the midsection suggests that the mass flux of tetrafluoroethane should be approximately 5%–10% lower than that of the freestream air to ensure that the injection is less intrusive to the flow field while still achieving the visibility required to characterize the flow field. For long-exposure photography, the reattachment length or the length scale behind the step can be estimated when the exposure time of a camera is 3.2–12 times longer than the timescale of vortex roll-up. For high-speed photography, the sequence of vortex shedding can be adequately visualized when the frame rate of a camera is at least four times higher than the vortex shedding frequency.