Kiran Bhaganagar | University of Texas at San Antonio (original) (raw)

Papers by Kiran Bhaganagar

The focus of this work is to demonstrate that the response of flow over a rough wall is sig- nifi... more The focus of this work is to demonstrate that the response of flow over a rough wall is sig- nificantly different for idealized and irregular roughness when the spacing between the roughness elements (k) is changed for a given height of the roughness (h). For this purpose, turbulence flow in a channel with walls covered with idealized and irregular roughness has been simulated where the k/h has been systematically varied in the range of 3–35. A detailed study of the flow statistics such as mean flow, root mean square of velocity and vorticity fluctuations was performed. For the idealized roughness both the inner and outer layer of the turbulent boundary layer are altered due to surface roughness, whereas, only the inner layer is altered due to an irregular roughness with similar geometrical parameters. Two distinct regimes that are evident for flow over idealized roughness based on streamwise spacing (k/h) can be unified using solidity ratio (measure of degree of sparseness) parameter. Irregular roughness exhibits no sensitivity to k/h parameter, sug- gesting that instead of solidity ratio, statistical parameters such as skewness and kurtosis of roughness distribution are more important to classify flow over irregular roughness.

Interactions between the nocturnal atmospheric boundary layer (ABL) and wind turbines (WTs) can b... more Interactions between the nocturnal atmospheric boundary layer (ABL) and wind turbines (WTs) can be complicated due to the presence of low level jets (LLJ), a region which creates wind speeds higher than geostrophic wind speed. A study has been performed to isolate the effect of mean forcings of the ABL on turbulence energetics and structures in the wake of WT. Large eddy simulation with an actuator line model has been used as a tool to simulate a full-scale 5-MW WT under two different realistic atmospheric states of the stable ABL corresponding to low- and high-stratification. The study clearly demonstrates that the large-scale forcings of thermally stratified atmospheric boundary characterized by shear- and buoyancy-driven turbulence significantly influence the wake structure of a wind turbine. For the WT in low-stratified ABL, the jets occur above the WT resulting in a strong mixed layer behind the WT. High turbulence results in a faster wake re- covery. For the WT in high-stratified ABL, the jets occur near the hub-height resulting in an asymmetric wake structure. The jets confine the mixing to hub- height resulting in a slower wake recovery. Vertical shear causes the interaction of the root- and lower-tip vortices resulting in the instability of the root vortex leading to an enhanced shear stress and turbulent kinetic energy. The tip vortices exhibit mutual inductance between adjacent vortex filaments resulting in vortex merging. LLJs are an important metric associated with mean atmospheric forcings that dic- tate the turbulence generated in WT wake and the wake recovery of a WT in a sta- ble ABL. VC 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4907687]

A fundamental study has been performed to understand the effect of unsteady forcing on turbulence... more A fundamental study has been performed to understand the effect of unsteady forcing on turbulence
statistics in channel flow with rough walls using direct numerical simulation. Unsteady flows have
been generated by applying an unsteady nonzero mean forcing in the form of time varying pressure
gradient such that the amplitude of oscillations is between 19% and 26% of mean centerline velocity
and covering a range of forcing frequencies. The analysis has revealed unsteady forcing, depending
on the forcing frequency, results in enhanced roughness compared to steady channel flow. The
rough-wall flow dynamics have been categorized into high-, intermediate-, and low-frequency
regimes. In the regime of high-frequency forcing, unsteadiness alters the mean velocity and
turbulence intensities only in the inner layer of the turbulent boundary layer. Further, the turbulence
intensities are out of phase with each other and also with the external forcing. In the regime of
intermediate-frequency forcing, mean velocity and turbulence intensities are altered beyond the
inner layer. In the inner layer, the turbulence intensities are out of phase with each other. The
Reynolds stress is in phase with the external forcing in the inner layer, but it is out of phase in the
outer layer. In the regime of low-frequency forcing, the mean velocity and turbulence intensities are
significantly altered throughout the turbulent boundary layer.

The paper deals with numerical investigation of the effect of plaque morphology on the flow chara... more The paper deals with numerical investigation of the effect of plaque morphology on the
flow characteristics in a diseased coronary artery using realistic plaque morphology. The
morphological information of the lumen and the plaque is obtained from intravascular
ultrasound imaging measurements of 42 patients performed at Cleveland Clinic Foundation,
Ohio. For this data, study of Bhaganagar et al. (2010) [1] has revealed the stenosis
for 42 patients can be categorized into four types – type I (peak-valley), type II (ascending),
type III (descending), and type IV (diffuse). The aim of the present study is to isolate the
effect of shape of the stenosis on the flow characteristics for a given degree of the stenosis.
In this study, we conduct fluid dynamic simulations for the four stenosis types (type I–IV)
and analyze the differences in the flow characteristics between these types. Finely refined
tetrahedral mesh for the 3-D solid model of the artery with plaques has been generated.
The 3-D steady flow simulations were performed using the turbulence (k–e) model in a
finite volume based computational fluid dynamics solver. The axial velocity, the radial
velocity, turbulence kinetic energy and wall shear stress profiles of the plaque have been
analyzed. From the axial and radial velocity profiles results the differences in the velocity
patterns are significantly visible at proximal as well as distal to the throat, region of maximum
stenosis. Turbulent kinetic energy and wall shear stress profiles have revealed significant
differences in the vicinity of the plaque. Additional unsteady flow simulations have
been performed to validate the hypothesis of the significance of plaque morphology in flow
alterations in diseased coronary artery. The results revealed the importance of accounting
for plaque morphology in addition to plaque height to accurately characterize the turbulent
flow in a diseased coronary artery.

Direct numerical simulation is used to understand the flow over ripple-shaped random rough elemen... more Direct numerical simulation is used to understand the flow over ripple-shaped
random rough elements. The random roughness has been generated using the model
for sand ripples consisting of saltation, creep and suspension processes. A set of
metrics based on both the geometrical and statistical properties of roughness was
derived to characterize random roughness. The eight cases that have been studied
with varied asymmetric distribution and “peakedness” as specified by the skewness
and kurtosis of the height distribution varying from 0.28 to 0.7, and from 1.8 to
2.2, respectively. Analogous to λ/h for canonical (regular) roughness, λavg/hmax was
selected as the geometrical parameter to characterize the surface, ranging from 4 to
26. The results have revealed that roughness significantly alters the mean velocity as
well as turbulence in the inner layer. The outer layer is relatively unaffected due to the
presence of roughness. The results further revealed that roughness distribution that is
symmetric and which is well spread out has a stronger influence on the mean flow in
the near-wall flow region. Altering the λavg/hmax affects the skewness and kurtosis of
the height distribution, which are the important metrics to quantify the mean flowover
a rough wall with random rough elements. The peak turbulence intensities increased
with increasing λavg/hmax, however its effect on rms of the velocity fluctuations was
confined to a region very close to the rough wall. Similar to mean flow, a symmetrical
and a well-spread roughness distribution resulted in more energetic turbulent
structures, stronger near-wall vortices, and increased turbulence activity. A similar
trend was observed for the small scale features of the turbulent flow as seen from
the vorticity fluctuations. To characterize turbulent flow over random rough surface
of given maximum height, the geometrical parameter λavg/hmax as well as statistical
parameters, the skewness and kurtosis are important metrics to be considered.

Direct numerical simulation (DNS) is used to simulate lock-exchange flow to understand density cu... more Direct numerical simulation (DNS) is used to simulate lock-exchange flow to understand density currents over rough surfaces. This work is one of
the first DNS to simulate density currents over rough walls. The simulations are performed at a Grashof number of 1.6 × 107. The non-dimensional
height of the roughness elements with respect to the half-height of the channel is 0.12. Roughness reduces the speed of the front. Furthermore, the
instabilities are significantly enhanced resulting in secondary instabilities that arise much earlier in time. Roughness introduces an additional vorticity
generation mechanism which is comparable to the vorticity generated from Kelvin–Helmholtz and lobe–cleft type of instabilities. Flow exhibits
significant differences near the leading edge or the nose of the front of the density currents due to roughness.

Turbulence structure in the wake behind a full-scale horizontal-axis wind turbine under the influ... more Turbulence structure in the wake behind a full-scale horizontal-axis wind turbine
under the influence of real-time atmospheric inflow conditions has been investigated using
actuator-line-model based large-eddy-simulations. Precursor atmospheric boundary layer
(ABL) simulations have been performed to obtain mean and turbulence states of the
atmosphere under stable stratification subjected to two different cooling rates. Wind
turbine simulations have revealed that, in addition to wind shear and ABL turbulence,
height-varying wind angle and low-level jets are ABL metrics that influence the structure
of the turbine wake. Increasing stability results in shallower boundary layers with stronger
wind shear, steeper vertical wind angle gradients, lower turbulence, and suppressed vertical
motions. A turbulent mixing layer forms downstream of the wind turbines, the strength and
size of which decreases with increasing stability. Height dependent wind angle and
turbulence are the ABL metrics influencing the lateral wake expansion. Further, ABL
metrics strongly impact the evolution of tip and root vortices formed behind the rotor.
Two factors play an important role in wake meandering: tip vortex merging due to the
mutual inductance form of instability and the corresponding instability of the turbulent
mixing layer

In order to profile the characteristics of turbulent flow in the presence of roughness, we perfor... more In order to profile the characteristics of turbulent flow in the presence of roughness, we perform direct-numerical-simulation (DNS) of a turbulent channel flow, where we introduce roughness using an immersed boundary method (IBM). Our previous analysis revealed the importance of aspect ratio of the roughness elements to determine the extent of communication between the inner- and outer-layers. Now, we explore the role of roughness density. In particular, we perform a series of sensitivity tests which permit a straight forward assessment of the effect of roughness density on the pressure distribution around the roughness elements. Using the sensitivity tests we desire to answer the following questions, (1) What is the correlation of the form drag with the roughness density? (2) Can we isolate roughness parameters to characterize the turbulent flow in the presence of roughness. (3) What is/are the flow characteristics which are the defining features of the presence of roughness.

During the early stages of atherosclerotic heart disease, fatty material accumulates in the coron... more During the early stages of atherosclerotic heart disease, fatty material accumulates in the coronary artery resulting in development of streaks of plaque and creating high levels of turbulence, and with significantly modified flow parameters. Diagnostic measures performed during this early stage may not show any evidence of coronary artery disease, because the lumen of the coronary artery has not decreased in caliber. These streaks do not obstruct the flow of blood but alter the flow characteristics, even at this preclinical stage. This talk presents the preliminary results for the analysis of turbulent flow characteristics for a range of atherosclerotic plaque configurations in the left main coronary artery. For this purpose a CAD/medical imaging based direct-simulation (DNS) tool has been developed. The Navier-stokes equations are solved in the vertical vorticity-velocity formulation. The plaque is introduced using immersed body technique. The geometric acquisition of the artery geometry and plaque morphology is obtained using CAD based commercial software.

Journal of Turbulence, 2007

Flow Turbulence and Combustion, 2004

Direct numerical simulation of turbulent incompressible plane-channel flow between a smooth wall ... more Direct numerical simulation of turbulent incompressible plane-channel flow between a smooth wall and one covered with regular three-dimensional roughness elements is performed. While the impact of roughness on the mean-velocity profile of turbulent wall layers is well understood, at least qualitatively, the manner in which other features are affected, especially in the outer layer, has been more controversial. We compare results from the smooth- and rough-wall sides of the channel for three different roughness heights of h += 5.4, 10.8, and 21.6 for Re τ of 400, to isolate the effects of the roughness on turbulent statistics and the instantaneous turbulence structure at large and small scales. We focus on the interaction between the near-wall and outer-layer regions, in particular the extent to which the near-wall behavior influences the flow further away from the surface. Roughness tends to increase the intensity of the velocity and vorticity fluctuations in the inner layer. In the outer layer, although the roughness alters the velocity fluctuations, the vorticity fluctuations are relatively unaffected. The higher-order moments and the energy budgets demonstrate significant differences between the smooth-wall and rough-wall sides in the processes associated with the wall-normal fluxes of the Reynolds shear stresses and turbulence kinetic energy. The length scales and flow dynamics in the roughness sublayer, the spatially inhomogeneous layer within which the flow is directly influenced by the individual roughness elements, are also examined. Alternative mechanisms involved in producing and maintaining near-wall turbulence in rough-wall boundary layers are also considered. We find that the strength of the inner/outer-layer interactions are greatly affected by the size of the roughness elements.

To get a good understanding of the effect of surface-roughness in altering the flow in a turbulen... more To get a good understanding of the effect of surface-roughness in altering the flow in a turbulent boundary layer it is important to understand the alterations in the dynamical activity of the flow. For this purpose direct proper orthogonal decomposition (POD) has been used as a tool. The data used for the POD has been obtained from direct numerical simulation of flow in a channel with egg-carton roughness elements. In this talk the effects of surface-roughness on the temporal flow dynamics such as bursting frequency of the energetic structures in the flow will be discussed. VITA detection technique has been used to obtain the bursting frequency. It has confirmed that rough-wall has a shorter bursting period and a higher turbulence activity compared to the smooth-wall. The results have confirmed the existence of roll and propagating modes for flow over rough-wall. In addition to the turbulent kinetic energy, the concept of entropy that has been introduced in this study within the context of degree of distribution of energy over range of scales, is a useful metric to categorize the rough-wall flow dynamics.

Direct numerical simulation of turbulent channel flow between a smooth wall and one covered with ... more Direct numerical simulation of turbulent channel flow between a smooth wall and one covered with three-dimensional roughness elements is performed to study the effects of surface roughness on turbulent boundary layers. While the impact of roughness upon the mean-velocity profile of turbulent wall layers is well understood, the manner in which other features are affected, especially in the outer layer, has been more controversial. Our results show that both statistics and turbulence structures in the outer layer are significantly affected by the presence of surface roughness. The higher-order moments and the energy budgets demonstrate significant differences between the smooth and rough sides in the wall-normal flux of the Reynolds shear stresses and turbulence kinetic energy. The length scales and flow dynamics in the roughness sublayer, the spatially inhomogeneous layer within which the flow is directly influenced by the individual roughness elements, are also examined. Our goal is to classify the effects of surface roughness in terms the following input parameters, such as roughness distribution and aspect ratio of the roughness elements in the streamwise and spanwise directions, and how these parameters affect the roughness length scales including the roughness sublayer. Possible alternative (non-smooth wall) mechanisms involved in producing and maintaining near-wall turbulence in rough-wall boundary layers are also examined.

In order to get a better understanding of the effects of surface roughness on turbulent boundary ... more In order to get a better understanding of the effects of surface roughness on turbulent boundary layers, direct numerical simulations of turbulent channel flow with a rough wall have been performed. An immersed-boundary approach has been used to emulate the effect of three-dimensional roughness with varying heights of h^+=5,10,20. This talk will focus on the effects of roughness on higher-order statistics, turbulent transport characteristics and a possible `different' vertical transport mechanism, and the manner in which the surface roughness influences the outer layer. Differences in the characteristics of the vertical-transport of the Reynolds stresses, and turbulent transport term in the energy budget have been observed. The normalized magnitudes of the strain rates and variance of vorticity and velocity-vorticity correlations show the roughness effects on the outer layer. The results from numerical experiments conducted to determine the effect of surface roughness on the self-regenerating mechanism in turbulent boundary layers will be presented.

![Research paper thumbnail of Investigation of `Vortex-Related' Structures of Transition Using DNS of Spatial Transition to Turbulence](https://a.academia-assets.com/images/blank-paper.jpg)

We focus on the main structures of the late stages of transition and beginning turbulence: the hi... more We focus on the main structures of the late stages of transition and beginning turbulence: the high shear-layer, the horseshoe-vortex, the streaky structure and the streamwise vortex. A model relating the above 'vortex-related' structures is proposed to explain the bursting process, using our DNS data-base for a turbulent boundary-layer. We define bursting as an event during which the low-speed fluid is ejected outward from the wall, generating the turbulence production in the boundary layer. During the burst, the high shear layer is an interface between the high-speed sweep upstream and the low-speed ejection downstream.

To get an improved understanding of wall-generated turbulence, a parallelized direct numerical si... more To get an improved understanding of wall-generated turbulence, a parallelized direct numerical simulation code has been developed for the spatial transition to turbulence in a boundary layer. The code is based on the three-dimensional Navier-Stokes equations for incompressible flow in vertical-velocity/vertical-vorticity formulation. For the spatial discretization fourth-order compact finite differences have been used, and the boundary conditions for the Laplacian of the vertical velocity are determined using an influence matrix method. For the outflow boundary, a buffer domain method in conjunction with parabolization of the Navier-Stokes equations has been used. The elliptic equations in this formulation are solved using a parallelized multigrid solver. Both the algorithmic and implementation scalability features have resulted in efficient parallelization. The validation of the DNS solver has been done both for linear and weakly nonlinear cases. The spatial process of laminar-turbulent transition has been simulated. The ultimate aim is to relate the characteristic structures and events of the turbulent boundary layer to well-known structures and events of transitional boundary layers such as Lambda-vortices and spikes.

Journal of Computational Physics, 2002

A highly accurate algorithm has been developed to study the process of spatial transition to turb... more A highly accurate algorithm has been developed to study the process of spatial transition to turbulence. The algorithmic details of the direct numerical simulation (DNS) of transition to turbulence in a boundary layer based on a formulation in terms of vertical velocity and vertical vorticity are presented. Issues concerning the boundary conditions are discussed. The linear viscous terms are discretized using an implicit Crank–Nicholson scheme, and a low-storage Runge–Kutta method is used for the nonlinear terms. For the spatial discretization, fourth-order compact finite differences have been used, as these have been found to have better resolution compared to explicit differencing schemes of comparable order. The number of grid points that are needed per wavelength is close to the theoretical optimum for any numerical scheme. The resulting time-discretized fourth-order equations are split up into two second-order equations, resulting in Helmholtz- and Poisson-type equations. The boundary conditions for the Laplacian of the vertical velocity are determined using an influence matrix method. A robust multigrid algorithm has been developed to solve the resulting anisotropic elliptical equations. For the outflow boundary, a buffer domain method, which smoothly reduces the disturbances to zero, in conjunction with parabolization of the Navier–Stokes equations has been used. The validation of the results for the DNS solver is made both for linear and weakly nonlinear cases.

In this talk we demonstrate Direct numerical simulation (DNS) as a robust and a valid tool to stu... more In this talk we demonstrate Direct numerical simulation (DNS) as a robust and a valid tool to study fundamental physics for coastal problems. We focus on turbulent pulsatile flow over 3-D ridged surfaces, which are relevant for oceanographic problems. We consider different morphological surfaces to explore the differences in turbulence production, dissipative and transport mechanisms. The influence of ridge shape and the pulse frequency on the scaling of the drag is explored.

Journal of Turbulence, 2007

Flow Turbulence and Combustion, 2004

![Research paper thumbnail of Investigation of `Vortex-Related' Structures of Transition Using DNS of Spatial Transition to Turbulence](https://a.academia-assets.com/images/blank-paper.jpg)

Journal of Computational Physics, 2002