A numerical investigation of laminar flow over a backward facing inclined step

Mushyam, Aditya; Bergadà, Josep M.; Nayeri, Christian Navid

doi:10.1007/s11012-015-0335-5

Cited by 10 publications

(5 citation statements)

References 21 publications

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“…These values for St are of the same order and the numerical difference between the two could be due to different roughness Reynolds numbers. It is worth noting that the Strouhal number values obtained for our roughness are comparable to St = 0.05 reported for an inclined backward facing step (without having roughness elements) by Mushyam et al (2016) at a comparable Reynolds number. These observations suggest that the distributed roughness CA introduces unsteady spanwise vortices into the boundary layer downstream of it.…”

Section: Time Averaged Piv Measurementssupporting

confidence: 82%

Characterization of streak development for boundary layer transition caused by isolated and distributed roughness

Joseph

Kumar

Diwan

2022

Preprint

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In this work, we characterize the early stages of transition in a flat-plate boundary layer caused by two types of surface roughness: distributed roughness (a strip of 24-grit emery) and isolated roughness (a cylindrical element). Towards this, we carry out extensive particle image velocimetry (PIV) measurements. The distributed roughness consists of a range of grit heights and introduces a broad range of disturbances downstream of it. In the pre-transitional region, both isolated and distributed roughness cause steady streaks, which for the latter case show a lack of spanwise symmetry. This is in contrast to the freestream turbulence (FST), which introduces unsteady streaks in the pre-transitional boundary layer. We propose tilting of spanwise vortices downstream of the distributed roughness to be the mechanism for generation of streamwise vortices, which are likely precursors to the onset of transition. For both the roughness configurations, the steady streaks develop instability resulting in localized streak breakdowns. The wall-normal PIV visualizations show streak instability features qualitatively similar to those for the FST-induced transition. For the distributed roughness, both the outer instability of lifted-up streaks and the inner instability due to streak interaction are present, whereas for the isolated roughness, the inner instability is dominant while the outer instability is much weaker. Conditional sampling of fluctuating velocity shows an asymmetry in the positive and negative fluctuations after the onset of transition, in a manner similar to that observed for the FST-induced transition. These observations suggest that the wall-normal distribution of unsteady velocity fluctuations in the early stages of transition is qualitatively similar irrespective of the source of disturbance (roughness/FST) causing transition. We expect that this commonality of features among different types of transition will be helpful in modelling flow over aerodynamic surfaces such as turbine blades and aircraft wings.

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Section: Time Averaged Piv Measurementssupporting

confidence: 82%

Characterization of streak development for boundary layer transition caused by isolated and distributed roughness

Joseph

Kumar

Diwan

2022

Preprint

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“…A similar value of Strouhal number (fk/U 𝑘 ) of 0.032 was reported in hotwire measurements conducted downstream of the same grit roughness at U ∞ =7.0 m/s in our results presented in Joseph et al [19]. We attributed the peak in the spectrum to vortices shed from the forward-backward step caused by the roughness strip consistent with previous studies such as Pinson and Wang [1], who conducted measurements downstream of a roughness strip placed at the leading edge, and Mushyam et al [30] who conducted numerical simulations downstream of a backward-facing step have reported Strouhal numbers of the same order. Note here that the frequency presented in the spectra in this work (across all configurations) is normalized by k (the height of the 24-grit roughness) instead of the local displacement thickness (𝛿*) since we are interested in understanding how the energy associated with the shedding frequency changes with the addition of the roughness.…”

Section: Single-strip Configurationsupporting

confidence: 90%

Delaying transition induced by a strip of distributed roughness using additional fine grit roughness

Joseph¹,

Kumar²,

Diwan³

2023

Preprint

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Distributed roughness occurs on aerodynamic surfaces like wind/gas turbine blades and aircraft wings causing early boundary layer transition resulting in higher skin friction, reduced lift-to-drag ratio, and lower power production. Despite being a recurring theme in practical engineering scenarios, the mechanism of boundary layer transition caused by distributed roughness is not well understood, and consequently, from a fluid dynamics perspective, methods for mitigating its effects are scarce. In this work, we present a passive method for delaying boundary layer transition caused by distributed roughness (simulated using a sandpaper strip) using a combination of secondary fine roughness strips placed immediately upstream and downstream of the distributed roughness. Hot-wire and PIV measurements are used to characterize the flow features and quantify transition delay. A combination of secondary roughness strips placed both upstream and downstream is shown to be most effective in delaying transition caused by the primary distributed roughness. Results suggest that the upstream roughness lifts the boundary layer reducing the effective Reynolds number of the primary roughness, while the downstream roughness reduces the strength of vortices shed from the primary roughness. A parametric study on the length and type of secondary roughness shows that smooth strips can also delay the transition and there is an optimal length of the secondary roughness beyond which increasing the extent of the downstream roughness has marginal effects on transition delay. Analysis of the higher moments of fluctuating velocity shows that there are no specific signatures in the flow corresponding to the secondary roughness after the onset of transition which suggests that the secondary roughness can delay transition without substantially altering the transitional flow features. The results point to an adaptable and practical method for in- *

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“…The distance between the end of the cathode and outlet boundary, where the Newman boundary condition is applied, was considered as 20 cm, indicated in Figure A.

\begin{matrix} lefttrue \\ u^{n + 1} & = \overset{true˜}{u} + u^{'} \\ v^{n + 1} & = \overset{true˜}{v} + v^{'} \\ p^{n + 1} & = \overset{true˜}{p} + p^{'} \end{matrix}

…”

Section: Description Of the Numerical Modelmentioning

confidence: 99%

“…MAC (Marker and Cell) method with velocity and pressure coupling was applied using a predictor‐corrector strategy. In this study, respective pressure correction factor in all the neighboring cells were considered, which are ignored in the original formulation . This is an improvement with respect to the original MAC method, as it minimizes the error involved in the calculation and achieves flow incompressibility relatively faster.…”

Section: Description Of the Numerical Modelmentioning

confidence: 99%

“…In this study, respective pressure correction factor in all the neighboring cells were considered, which are ignored in the original formulation. 46,48,49 This is an improvement with respect to the original MAC method, as it minimizes the error involved in the calculation and achieves flow incompressibility relatively faster. The velocity and pressure correction factors presented in Equation (27) are substituted in respective momentum Equations (18)- (19) to derive the correlation between pressure correction factors of velocity and pressure presented in Equation (28).…”

Section: Numerical Solution and Necessary Boundary Conditionsmentioning

confidence: 99%

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A plasma‐fluid model for EHD flow in DBD actuators and experimental validation

Mushyam

Rodrigues

Páscoa

2019

Numerical Methods in Fluids

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Summary A numerical method to simulate plasma induced electrohydrodynamic flow is proposed in this study. The numerical model consists of three components. Firstly, a potential module to simulate temporal potential and electric field generated in the ionized fluid. Secondly, a plasma module to simulate plasma development and charge particle densities. Finally, a fluid module to simulate the flow affected by the body forces induced by the movement of the charged particles. Fluid flow is modeled using modified predictor‐corrector strategy as proposed in the marker and cell method. The velocity field was corrected to achieve incompressible flow by calculating pressure correction factors, considered in all cells. Numerical convergence and time sensitivity analysis were carried to confirm grid independence and determine an efficient time step for simulations. Numerical computations are validated by comparing with experimental results of discharge currents and current densities. They were found to be in very good agreement thus providing an extensive validation. Furthermore, quiescent flow over a dielectric barrier discharge actuator is simulated in the this study, using the proposed plasma‐fluid model, to model flow evolution and resolve temporal flow features for detailed analysis. The streamline and vorticity plots were analyzed and compared with experimental results, and flow results were found to be in‐line with the experiments.

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A numerical investigation of laminar flow over a backward facing inclined step

Cited by 10 publications

References 21 publications

Characterization of streak development for boundary layer transition caused by isolated and distributed roughness

Characterization of streak development for boundary layer transition caused by isolated and distributed roughness

Delaying transition induced by a strip of distributed roughness using additional fine grit roughness

A plasma‐fluid model for EHD flow in DBD actuators and experimental validation

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