Forward-facing Step Research Articles

Impact of a forward-facing step on the development of crossflow instability

Abstract

Fluorescent PIV using Atomized Liquid Particles

It is shown that aerosolized fluorescent particles generated using a Venturi-type atomizer, from a solution of fluorescent Kiton Red 620 dye in a water/glycol fluid, provide effective flow seeding for fluorescent PIV. The atomized liquid particles were found to be of acceptable size for PIV purposes, with 92% of detected particles by number concentration measuring < 1 μm in diameter. A PIV application was conducted in a wind tunnel (freestream velocity U∞ = 27 m/s), using the particles for measurement of the boundary layer flow approaching a forward-facing step (approach boundary layer momentum thickness Reynolds number of Reθ = 5930), to identify potential benefits in near-wall regions normally affected by unwanted laser reflections from tunnel surfaces. Particles were generated from solutions with dye molar concentrations of 2.5 × 10−3 and 1.0 × 10−2 mol/L, and PIV images were obtained for both elastic Mie scattering and filtered, Stokes-shifted fluorescent light. Raw images indicate that the fluorescence yield of the 1.0 × 10−2 mol/L solution provides PIV images with high contrast, even in the near-surface regions where Mie scattering images are highly affected by surface reflections. Boundary layer profiles are processed in the adverse pressure gradient region leading up to the forward-facing step, where the fluorescent PIV performed comparably to the most optimized Mie scattering PIV; both obtained data as near to the wall as 30 μm, or 2 viscous wall units in our flow of interest. These results indicate that the new seeding method holds excellent promise for near-surface measurement applications with more complicated three-dimensional geometries, where it is impossible to arrange PIV cameras to reject surface-scattered light.

Augmentation of Heat Transfer and AL2O3-Nanofluid Flow Over Vertical Double Forward-Facing Step (DFFS)

Nanofluids are recommended to improve heat transfer in cooling and heating systems, resulting in significant benefits. This paper numerically investigates turbulent heat transfer and Al2O3-nanofluid flow over a vertical double forward-facing step. A two dimensional with three different cases of vertical DFFS is conducted using K-ɛ model based on finite volume method for volume fraction of nanofluids varied for 1%, 2% ,3% and Reynolds number changed from 10000 to 40000. With increasing Reynolds number, there is an increase in local coefficients of heat transfer, with the highest coefficient of heat transfer detected at Re=40000. For volume fractions of Al2O3= 3% and Reynolds numbers of 40000, the effects of step height on surface coefficients of heat transfer are described. In addition, the findings have discovered that as the volume fraction of Al2O3 nanofluids has increased, the coefficient of heat transfer has increased as well, with the maximum coefficient of heat transfer occurring at a volume fraction of Al2O3 nanofluids of 3%. Furthermore, the first step-case 2 local coefficient of heat transfer has been higher than the first step-cases 1 and 3. Increased Re number causes a sharp drop in local static pressure at the first and at the second steps. Due to the recirculation flow, there has been a reduction in velocity profile near the first and second steps, indicating an increase in heat transfer rate. Moreover, velocity counters are shown in order to demonstrate how Reynolds number affects the size of the recirculation zone. In addition, the turbulence kinetic energy counter has been shown in order to demonstrate how to achieve thermal efficiency in the second step in all the cases.

Mixed convection in a channel with buoyancy force over backward and forward facing steps: The effects of inclination and geometry

This paper presents the computational results of heat transfer for a 2D laminar flow with different channel tilts with forward facing step and backward facing step, taking into account buoyancy forces for various bottom wall lengths. The inclination angle influence on the distribution of velocity and temperature is investigated. The validated numerical algorithm was used to the problem forward and backward facing steps with buoyancy force and at various tilt angles. From the obtained numerical results, it can be noticed that the length of the lower part of the channel has a very strong effect on the flow fluctuation and temperature distribution over the entire channel. It should be noticed that the tilt angle also has a very strong effect on the distribution of flow and temperature. Thus, taking into account the buoyancy force changes the shape of the main recirculation region, but at the same time, regardless of the different tilt angles, the number of vortices does not change, but only the size of the vortices changes. It should also be noticed that when the buoyancy force is taken into account, cooling occurs more efficiently in the middle of the channel.

Computations of Turbulent Flow Characteristics and Heat Transfer on Forward-Facing Steps.(Dept.M)

Computations of Turbulent Flow Characteristics and Heat Transfer on Forward-Facing Steps.(Dept.M)

Three-dimensional structural characteristics of flow separation induced by a forward-facing step in a turbulent channel flow

Abstract

Flow topology and bifurcations of buoyancy and mixed convection in an elongated channel with an abrupt section variation

A forward-facing step (FFS) located half-way in an elongated duct subjected to a horizontal pressure gradient and a vertical temperature difference is considered as a simplified geometrical model to investigate numerically typical problems of internal non-isothermal flow in the presence of blunt obstacles. The sensitivity of this system to thermal buoyancy for each considered rate of fluid injection (measured through the related Richardson number, Ri) is explored by varying parametrically the corresponding Rayleigh number (Ra) over a large interval of orders of magnitude (up to the onset of chaos) and assuming two alternate paradigms for the bottom of the considered channel, namely an adiabatic or kept-at-constant temperature (hot) boundary. Through this conceptual framework, a kaleidoscope of situations are revealed in the (Ri, Ra) space, differing in terms of flow patterning behavior, thermal plume generation phenomena, intensity of heat exchange at the walls and bifurcation scenario. In particular, while for the isothermal floor case a higher Ri leads to an increase in the value of the Rayleigh number needed for transition to time-dependent flow, the corresponding trend becomes non-monotonic if the same boundary is thermally insulated. In such a case the Nusselt number (Nu) calculated for the horizontal surface of the step is always smaller than the equivalent Nu evaluated for the vertical side. The latter is significantly lowered when the hot-floor condition is assumed.

The Effects of Upstream Wall Roughness on the Spatio-Temporal Characteristics of Flow Separations Induced by a Forward-Facing Step

Abstract Separating and reattaching turbulent flows induced by a forward-facing step submerged in thick oncoming turbulent boundary layers (TBL) developed over smooth and rough upstream walls were investigated using time-resolved particle image velocimetry. The examined upstream walls resulted in smooth, transitionally rough, and fully rough wall conditions. The upstream boundary layer thicknesses were 4.3 and 6.7 times the step height in the smooth and rough wall cases, respectively. The Reynolds number based on the step height and freestream velocity was 7800. The effects of upstream wall roughness on the mean flow characteristics, Reynolds stresses defined in both Cartesian and curvilinear coordinate systems, as well as the unsteadiness of the turbulent separation bubbles were critically examined. The results show that upstream wall roughness increases the boundary layer thickness and turbulence intensity and consequently, promotes early mean flow reattachment over the step. Distinct regions of significantly elevated vertical Reynolds normal stress and Reynolds shear stress were observed upstream of the step in the fully rough wall case compared to the smooth wall case. Proper orthogonal decomposition (POD) and the reverse flow area over the step were employed to investigate the unsteadiness of the separation bubbles. The first POD mode coefficient and the reverse flow area over the step were strongly correlated and exhibited the same dominant frequency.

Turbulent flow characterization near the edge of a steep escarpment

Characteristics of turbulent flow over a steep escarpment were studied using a 1:25 scale model of the Bolund hill in a wind tunnel under neutral atmospheric stability conditions. The influence of Reynolds number, inflow shear profile, and escarpment geometry were investigated in relation to flow behavior including separation and reattachment, Reynolds stresses, vorticity, Turbulent Kinetic Energy (TKE) production, and Gaussianity. Multi-camera Particle Image Velocimetry (PIV) was used to examine the flow in the vicinity of the escarpment leading edge. Four test cases were studied across a Reynolds number range of 1 × 10 5 < R e h < 5 × 10 5 . Two different inflow profiles were evaluated, as well as two different leading edge geometries – a round edge with non-dimensional radius of curvature a η ≈ 0.2 and a sharp edge with η ≈ 0 . A sharp leading edge was found to produce a well-defined separation bubble, yielding distributions and magnitudes of Reynolds stress, vorticity and TKE production closely resembling those of flow over an idealized forward facing step, as well as full-scale measurements of TKE. Gaussianity of the flow was significantly modified by the escarpment, with high skewness and kurtosis occurring in the sharp edge case along the boundary of the separation bubble. • Small changes in escarpment leading edge sharpness greatly influence turbulent flow behavior. • Sharp edge caused relocation of area of peak turbulent kinetic energy. • Turbulent flow of sharp edge case better approximated full-scale measurements. • Turbulent flow of sharp edge case similar to canonical forward-facing step. • Gaussianity modified along separation bubble boundary.

Laminar Flow Control and Drag Reduction using Biomimetically Inspired Forward Facing Steps

This paper explores the use of shark-skin inspired two-dimensional forward facing steps to attain laminar flow control, delay boundary layer transition and to reduce drag. Computation Fluid Dynamics (CFD) simulations are carried out on strategically placed forward facing steps within the laminar boundary layer using the Transition SST model in FLUENT after comprehensive benchmarking and validation of the simulation setup. Results presented in this paper indicate that the boundary layer thickness to step height ratio (d/h), as well as the location of the step within the laminar boundary layer (x/L), greatly influence transition onset. The presence of a strategically placed forward facing step within the laminar boundary layer might damp disturbances within the laminar boundary layer, reduce wall shear stress and energize the boundary layer leading to transition onset delay and drag reduction as compared to a conventional flat plate. Results presented in this paper indicate that a transition delay of 20% and a drag reduction of 6% is achievable, thereby demonstrating the veracity of biomimicry as a potential avenue to attain improved aerodynamic performance.

Direct numerical simulation of TS-waves over suction panel steps from manufacturing tolerances

To determine allowable tolerances between successive suction panels at hybrid laminar wings with suction surfaces, direct numerical simulations of Tollmien–Schlichting waves over different steps are carried out for realistic suction rates on a wind tunnel configuration. Simulations at given suction panel positions over forward and backward facing steps are carried out by the use of a high-order method for the direct simulation of Tollmien–Schlichting wave growth. Comparisons between high-fidelity direct numerical simulations and quick linear stability calculations have shown capabilities and limits of the well-validated linear stability theory design approach.

Analysis of viscoelastic non-Newtonian fluid over a vertical forward-facing step using the Maxwell fractional model

In this study, unsteady of the fluid flow and heat transfer of natural convection of viscoelastic non-Newtonian fluid have been investigated with the Maxwell fractional model on a vertical forward-facing step. Fractional derivatives are used to the fluid properties more accurate calculations. The fractional time derivative of the Caputo method and the fractional derivative of the velocity and temperature of the viscoelastic fluid are calculated using the finite difference numerical method by combining the L-1 algorithm. The effect of various factors such as parameters of velocity and temperature fraction derivative (α and β), buoyancy force, step length, and Prandtl number on velocity profile, the temperature of the viscoelastic fluid, Nusselt number and friction coefficient on a vertical forward-facing step are investigated. Numerical results show that with increasing α, β and step length, the coefficient of friction increases and also decreases with increasing Prandtl. With increasing α, the average Nusselt value decreases, and with increasing β, Prandtl, and step length increases. It is also observed that the thickness of the momentum and thermal boundary layer in viscoelastic fluids is higher than that of the Newtonian fluid boundary layer.

Sound radiated from low Mach number turbulent boundary layer flows（Turbulent boundary layer on a smooth plate and over a small forward facing step）

High-resolution numerical simulation has been performed to study aeroacoustic noise radiated from a turbulent boundary layer at freestream Mach number Ma = 0.3, which develops on a smooth flat plate and over a small forward-facing step. Sound waves radiated from the turbulent boundary layer on the flat plate are dominant in a very-low-frequency band and have characteristics of a linear sound source. The sound waves are observed in the far-field from the boundary propagate outside of the flow region where small hydrodynamic pressure fluctuations with a significant long-wavelength appear. The instantaneous hydrodynamic pressure fields that gradually develop or decay while moving downstream with the turbulent boundary layer are associated with the evolution of various vortical structures. The characteristics of the sound wave being a linear sound source and dominant in a very-low-frequency band are similar to those of the real high-speed train. The sound waves generated from the turbulent boundary layer over the forward-facing step with a height (SH) of SH/y+ ≅ 62 are significantly larger in a full frequency band than those radiated from the turbulent boundary layer on the flat plate. Numerical results of the turbulent boundary layer over the forward-facing step with a height of SH/y+ ≅ 30 show that the sound waves are dominant in the low-frequency band, and also step-specific sound waves are generated in the high-frequency band. Further, even if the step height is SH/y+ ≅ 7.5, the sound waves unique to the small step are generated in the high-frequency band.

Hybrid Al2O3-Cu/water nanofluid flow and heat transfer over vertical double forward-facing step

Turbulent heat transfer and hybrid Al2O3-Cu-nanofluid over vertical double forward facing-step is numerically conducted. The k-? standard model based on finite volume method in 2-D are applied to investigate the influences of Reynolds number, step height, volume fractions hybrid Al2O3-Cu-nanofluid on thermal performance. In this paper, different step heights for three cases of vertical double forward-facing step are adopted by five different of volume fractions of hybrid (Al2O3-Cu-water) nanofluid varied for 0.1, 0.33, 0.75, 1, and 2, while the Reynolds number different between 10000 to 40000 with temperature is constant. The main findings revealed that rise in local heat transfer coefficients with raised Reynolds number and maximum heat transfer coefficient was noticed at Re = 40000. Also rises in heat transfer coefficient detected with increased volume concentrations of hybrid (Al2O3-Cu-water) nanofluid and the maximum heat transfer coefficient found at hybrid Al2O3-Cu-water nanofluid of 2% in compared with others. It is also found that rise in surface heat transfer coefficient at 1st step-Case 2 was greater than at 1st step-Case 1 and 3 while was higher at 2nd step-Case 3. Average heat transfer coefficient with Reynolds number for all cases are presented in this paper and found that the maximum average heat transfer coefficient was at Case 2 compared with Case 1 and 3. Gradually increases in skin friction coefficient remarked at 1st and 2nd steps of the channel and drop in skin friction coefficient was obtained with increased of Reynolds number. Counter of velocity was presented to show the re-circulation regions at 1st and 2nd steps as clarified the enrichment in heat transfer rate. Furthermore, the counter of turbulence kinetic energy contour was displayed to provide demonstration for achieving thermal performance at second step for all cases.

Direct numerical simulation of turbulent flow separation induced by a forward-facing step

Direct numerical simulation is used to investigate turbulent flow separations generated by a forward-facing step exposed to an incoming fully-developed turbulent plane channel flow at Reτ=180. The step height is 25% of the inlet channel height. The results are analyzed in terms of the topology of the Reynolds stresses, transport of turbulence kinetic energy (TKE) and unsteadiness of separation bubbles using a broad range of post-processing techniques, including the Lumley triangle, quadrant analysis, auto-correlation, joint probability density function, proper orthogonal decomposition and linear stochastic estimation. The incoming turbulence intensity is elevated as the step is approached and peaks along the mean separating streamline over the step. The topology of the Reynolds stresses switches from two-component to axisymmetric along the mean separating streamline over the step. The production term of TKE peaks in the separated shear layer, while the turbulent diffusion term tends to transport TKE away from the shear layer. An expanded separation bubble upstream of the step is typically associated with a contracted separation bubble over the step. This pattern is captured by the first proper orthogonal decomposition (POD) mode. In the first POD mode, the streamwise and vertical fluctuating velocities switch signs abruptly near the mean separating streamline over the step. This is characterized by two pairs of counter-rotating vortices: one pair of counter-rotating vortices leaning over the step and another pair of opposite-signed counter-rotating vortices near the top surface of the step.

Impact of Topographic Steps in the Wake and Power of a Wind Turbine: Part A—Statistics

We experimentally explored the modulation of various forward- and backward-facing topographic steps on the wake and power output of a wind turbine model. The sharp surface changes located in the vicinity of the turbine tower consisted of steps Δz0/dT=−0.64, −0.42, −0.21, 0, 0.21, and 0.42, where Δz0 is the level difference between the upwind and downwind sides of the step and dT is the turbine diameter. Particle image velocimetry was used to obtain the wake statistics in the wake within the streamwise distance x/dT∈[2, 5] and vertical span z/dT∈[−0.7, 0.7], where the origin is set at the rotor hub. Complementary single-point hotwire measurements were obtained in the wake along the rotor axis every Δx/dT=1 within x/dT∈[1, 8]. Mean power output and its fluctuations were obtained for each of the six scenarios. The results indicate strong modulation of the steps in the wake statistics and some effect on the power output. Remarkably, the backward-facing steps induced a larger velocity deficit in the wake with respect to the base case with substantial wake deflection. In contrast, the forward-facing steps exhibited a much lower velocity deficit and negligible wake deflection. The mean flow and velocity gradients’ changes promoted distinct turbulence dynamics and, consequently, associated levels. In particular, turbulence intensity and kinematic Reynolds shear stress were enhanced and reduced with the backward- and forward-facing steps, respectively. It is worth pointing out the particular effect of the steps on the transport of the turbulence kinetic energy TKE. Ejections were predominant around the top tip, whereas sweeps dominated around the turbine hub height. The magnitude of these quantities was sensitive to the step height. In particular, a much weaker sweep occurred in the forward-facing steps; in addition, the flat terrain and the backward-facing step cases shared strong sweeps.

Flow boiling heat transfer and associated bubble behaviors over backward- and forward-facing steps

The influence of backward-facing step and forward-facing step on flow boiling heat transfer performance was experimentally studied. The experiments were carried out with expansion ratio of 2, 2.5 and 3, and channel Reynolds number of 3048 to 6097. The heated surface was immobilized at the downstream next to the step. FC-72 was used as the working fluid with the inlet subcooling of 35 K. The bubble behaviors were analyzed by the high-speed camera. The results reveal that the backward-facing step geometry has a negative effect on the heat transfer performance compared with the flat channel. The effect of forward-facing step geometry is related with the dimension of the recirculation regions: the heat transfer performance is enhanced if the reattachment point falls on the heated surface, and is deteriorated if the recirculation region is larger than the length of the heated surface. The bubble behaviors were analyzed to illustrate the dimension of the recirculation region and the mechanism of flow boiling heat transfer over a step.

The Impact of Streamwise Steps and Cavities Geometry on Supersonic Turbulent Boundary Layer

In some cases, the nozzle of a rocket engine could deform during manufacturing or testing, flare up and distort during operation in such a way that periodically repeating structures of depressions and protrusions form on the wall surface. Complex configurations of roughness topologies have been experimentally observed to form due to this fluid-structure interaction and have been known to distort the boundary layer and increase local drag and heat transfer. A numerical experiment was carried out using ANSYS FLUENT to assess the effects of geometry (streamwise wavelengths ? and length-to-depth ratios (L/D)) of discrete roughness topologies in the form of steps and cavities on flow field distortions and resulting total drag. It was found that cavity L/D ratios of 19.8 – 23.1 and 12.8 – 15.5 yielded the highest drag coefficients. The flow fields over these periodic arrays exhibited closed and a combination of closed cavity type features, and it can be concluded that the flow phenomena associated with separation caused by forward facing steps in closed cavity type topologies contributed the most to the total drag. L/D ratios of 6.3 – 8.4 exhibited the lowest drag coefficient. The lower drag coefficient is due to the modification of the turbulent boundary layer by the trapped coherent vortical structure within the cavities. Pressure forces become more dominant as the surface wavelength is decreased. For all cases, there was a pronounced increase in the boundary layer thickness after each successive cavity.

Vortical structures and density fluctuations analysis of supersonic forward-facing step controlled by self-sustaining dual synthetic jets

Shock wave/boundary layer interaction (SWBLI) is still one of the unresolved bottlenecks that restrict the development of more advanced flight vehicles. Supersonic forward-facing step (FFS), an extreme case of compression ramp, often occurs severe SWBLIs with a large separation bubble. In this paper, experimental investigations on vortical structures and density fluctuations characteristics of supersonic FFS controlled by self-sustaining dual synthetic jets (SDSJ) are carried out in a Mach number 2.95 wind tunnel. High spatial–temporal resolution flowfield images of FFS without/with active flow control are captured by adopting nano-particle-based planar laser scattering technique. The control effects of the distance between the actuator and the step are mainly compared. The paper finds that the SDSJ can effectively change the feature of flowfield, eliminate the separation shock and the reattachment shock, compel the original shock induced by the step leading edge to distort and reduce its intensity finally. Density fluctuations analysis demonstrates that the whole flows seem to move upstream with the increase of distance (dS-J). Discrete Fourier transformation spectrums results reveal that the fluctuations are mainly located in the low-frequency region at first. High-frequency components and frequency bandwidth increase slightly after the SDSJ are applied.

Natural convective flow and heat transfer on unconfined isothermal zigzag-shaped ribbed vertical surfaces

Natural convective heat transfer is commonly used as heat transfer mechanism in applications with low heat flux due to its reliability and cost effectiveness. In this study, we introduce zigzag-shaped ribs to vertical, isothermal, heated surfaces with the purpose of increasing natural convective heat transfer. The ribs are characterised by a rib height h, rib length p and vertical pitch distance L. We perform numerical simulations using the Boussinesq approximation by prescribing a linear density-temperature relation and investigate how changes in rib length p/L, rib height h/L affect heat transfer at GrL= 105 and GrL = 106 at Pr = 0.71.The results show how geometric variations affect heat transfer locally. Generally, local heat transfer increases along each outward-facing section and peaks at the tip of each rib. In the limiting case when surface approaches a forward facing step (e.g. p/L = 1), a significant decrease in heat transfer is observed on the horizontal section.A peak in heat transfer is observed for geometries with high rib lengths p/L = 0.9, where the surface-averaged Nusselt number is increased by 4.43% compared to the flat surface. This increases to 11.60% when correcting for the increase in surface area.