Abstract: The presence of bubbles in the boundary layer introduces corrections into the log law, which must be taken into account. In this work, a logarithmic wall law was presented for bubbly two phase flows. The wall law presented in this work was based on the postulation of additional turbulent viscosity associated with bubble wakes in the boundary layer. The presented wall law contained empirical constant accounting both for shear induced turbulence interaction and for non-linearity of bubble. This constant was deduced from experimental data. The wall friction prediction achieved with the wall law was compared to the experimental data, in the case of a turbulent boundary layer developing on a vertical flat plate in the presence of millimetric bubbles. A very good agreement between experimental and numerical wall friction prediction was verified. The agreement was especially noticeable for the low void fraction when bubble induced turbulence plays a significant role.
Abstract: The magnetohydrodynamic (MHD) Falkner-Skan
equations appear in study of laminar boundary layers flow over
a wedge in presence of a transverse magnetic field. The partial
differential equations of boundary layer problems in presence of
a transverse magnetic field are reduced to MHD Falkner-Skan
equation by similarity solution methods. This is a nonlinear ordinary
differential equation. In this paper, we solve this equation via
spectral collocation method based on Bessel functions of the first
kind. In this approach, we reduce the solution of the nonlinear
MHD Falkner-Skan equation to a solution of a nonlinear algebraic
equations system. Then, the resulting system is solved by Newton
method. We discuss obtained solution by studying the behavior
of boundary layer flow in terms of skin friction, velocity, various
amounts of magnetic field and angle of wedge. Finally, the results
are compared with other methods mentioned in literature. We can
conclude that the presented method has better accuracy than others.
Abstract: This paper will explore formation of HCl aerosol at atmospheric boundary layers and encourages the uptake of environmental modeling systems (EMSs) as a practice evaluation of gaseous emissions (“framework measures”) from small and medium-sized enterprises (SMEs). The conceptual model predicts greenhouse gas emissions to ecological points beyond landfill site operations. It focuses on incorporation traditional knowledge into baseline information for both measurement data and the mathematical results, regarding parameters influence model variable inputs. The paper has simplified parameters of aerosol processes based on the more complex aerosol process computations. The simple model can be implemented to both Gaussian and Eulerian rural dispersion models. Aerosol processes considered in this study were (i) the coagulation of particles, (ii) the condensation and evaporation of organic vapors, and (iii) dry deposition. The chemical transformation of gas-phase compounds is taken into account photochemical formulation with exposure effects according to HCl concentrations as starting point of risk assessment. The discussion set out distinctly aspect of sustainability in reflection inputs, outputs, and modes of impact on the environment. Thereby, models incorporate abiotic and biotic species to broaden the scope of integration for both quantification impact and assessment risks. The later environmental obligations suggest either a recommendation or a decision of what is a legislative should be achieved for mitigation measures of landfill gas (LFG) ultimately.
Abstract: The exploitation of flow pulsation in micro- and
mini-channels is a potentially useful technique for enhancing cooling
of high-end photonics and electronics systems. It is thought that
pulsation alters the thickness of the hydrodynamic and thermal
boundary layers, and hence affects the overall thermal resistance
of the heat sink. Although the fluid mechanics and heat transfer
are inextricably linked, it can be useful to decouple the parameters
to better understand the mechanisms underlying any heat transfer
enhancement. Using two-dimensional, two-component particle image
velocimetry, the current work intends to characterize the heat transfer
mechanisms in pulsating flow with a mean Reynolds number of
48 by experimentally quantifying the hydrodynamics of a generic
liquid-cooled channel geometry. Flows circulated through the test
section by a gear pump are modulated using a controller to achieve
sinusoidal flow pulsations with Womersley numbers of 7.45 and
2.36 and an amplitude ratio of 0.75. It is found that the transient
characteristics of the measured velocity profiles are dependent on the
speed of oscillation, in accordance with the analytical solution for
flow in a rectangular channel. A large velocity overshoot is observed
close to the wall at high frequencies, resulting from the interaction
of near-wall viscous stresses and inertial effects of the main fluid
body. The steep velocity gradients at the wall are indicative of
augmented heat transfer, although the local flow reversal may reduce
the upstream temperature difference in heat transfer applications.
While unsteady effects remain evident at the lower frequency, the
annular effect subsides and retreats from the wall. The shear rate at
the wall is increased during the accelerating half-cycle and decreased
during deceleration compared to steady flow, suggesting that the flow
may experience both enhanced and diminished heat transfer during
a single period. Hence, the thickness of the hydrodynamic boundary
layer is reduced for positively moving flow during one half of the
pulsation cycle at the investigated frequencies. It is expected that the
size of the thermal boundary layer is similarly reduced during the
cycle, leading to intervals of heat transfer enhancement.
Abstract: This paper presents a complete dynamic modeling
of a membrane distillation process. The model contains two
consistent dynamic models. A 2D advection-diffusion equation
for modeling the whole process and a modified heat equation
for modeling the membrane itself. The complete model describes
the temperature diffusion phenomenon across the feed, membrane,
permeate containers and boundary layers of the membrane. It gives
an online and complete temperature profile for each point in the
domain. It explains heat conduction and convection mechanisms that
take place inside the process in terms of mathematical parameters, and
justify process behavior during transient and steady state phases. The
process is monitored for any sudden change in the performance at any
instance of time. In addition, it assists maintaining production rates
as desired, and gives recommendations during membrane fabrication
stages. System performance and parameters can be optimized
and controlled using this complete dynamic model. Evolution of
membrane boundary temperature with time, vapor mass transfer along
the process, and temperature difference between membrane boundary
layers are depicted and included. Simulations were performed over
the complete model with real membrane specifications. The plots
show consistency between 2D advection-diffusion model and the
expected behavior of the systems as well as literature. Evolution
of heat inside the membrane starting from transient response till
reaching steady state response for fixed and varying times is
illustrated.
Abstract: The steady coupled dissipative layers, called
Marangoni mixed convection boundary layers, in the presence of a
magnetic field and solute concentration that are formed along the
surface of two immiscible fluids with uniform suction or injection
effects is examined. The similarity boundary layer equations are
solved numerically using the Runge-Kutta Fehlberg with shooting
technique. The Marangoni, buoyancy and external pressure gradient
effects that are generated in mixed convection boundary layer flow
are assessed. The velocity, temperature and concentration boundary
layers thickness decrease with the increase of the magnetic field
strength and the injection to suction. For buoyancy-opposed flow, the
Marangoni mixed convection parameter enhances the velocity
boundary layer but decreases the temperature and concentration
boundary layers. However, for the buoyancy-assisted flow, the
Marangoni mixed convection parameter decelerates the velocity but
increases the temperature and concentration boundary layers.
Abstract: Flow field around hypersonic vehicles is very
complex and difficult to simulate. The boundary layers are squeezed
between shock layer and body surface. Resolution of boundary layer,
shock wave and turbulent regions where the flow field has high
values is difficult of capture. Detached eddy simulation (DES) is a
modification of a RANS model in which the model switches to a
subgrid scale formulation in regions fine enough for LES
calculations. Regions near solid body boundaries and where the
turbulent length scale is less than the maximum grid dimension are
assigned the RANS mode of solution. As the turbulent length scale
exceeds the grid dimension, the regions are solved using the LES
mode. Therefore the grid resolution is not as demanding as pure LES,
thereby considerably cutting down the cost of the computation. In
this research study hypersonic flow is simulated at Mach 8 and
different angle of attacks to resolve the proper boundary layers and
discontinuities. The flow is also simulated in the long wake regions.
Mesh is little different than RANS simulations and it is made dense
near the boundary layers and in the wake regions to resolve it
properly. Hypersonic blunt cone cylinder body with frustrum at angle
5o and 10 o are simulated and there aerodynamics study is performed
to calculate aerodynamics characteristics of different geometries. The
results and then compared with experimental as well as with some
turbulence model (SA Model). The results achieved with DES
simulation have very good resolution as well as have excellent
agreement with experimental and available data. Unsteady
simulations are performed for DES calculations by using duel time
stepping method or implicit time stepping. The simulations are
performed at Mach number 8 and angle of attack from 0o to 10o for
all these cases. The results and resolutions for DES model found
much better than SA turbulence model.
Abstract: Numerical studies on race car aerodynamics at wing
in ground effect have been carried out using a steady 3d, double
precision, pressure-based, and standard k-epsilon turbulence model.
Through various parametric analytical studies we have observed that
at a particular speed and ground clearance of the wings a favorable
negative lift was found high at a particular angle of attack for all the
physical models considered in this paper. The fact is that if the
ground clearance height to chord length (h/c) is too small, the
developing boundary layers from either side (the ground and the
lower surface of the wing) can interact, leading to an altered variation
of the aerodynamic characteristics at wing in ground effect. Therefore
a suitable ground clearance must be predicted throughout the racing
for a better performance of the race car, which obviously depends
upon the coupled effects of the topography, wing orientation with
respect to the ground, the incoming flow features and/or the race car
speed. We have concluded that for the design of high performance
and high speed race cars the adjustable wings capable to alter the
ground clearance and the angles of attack is the best design option for
any race car for racing safely with variable speeds.
Abstract: In this work, we try to find the best setting
of Computational Fluid Dynamic solver available for the problems in
the field of supersonic internal flows. We used the supersonic air-toair
ejector to represent the typical problem in focus. There are
multiple oblique shock waves, shear layers, boundary layers
and normal shock interacting in the supersonic ejector making this
device typical in field of supersonic inner flows. Modeling of shocks
in general is demanding on the physical model of fluid, because
ordinary conservation equation does not conform to real conditions in
the near-shock region as found in many works. From these reasons,
we decided to take special care about solver setting in this article by
means of experimental approach of color Schlieren pictures and
pneumatic measurement. Fast pressure transducers were used to
measure unsteady static pressure in regimes with normal shock in
mixing chamber. Physical behavior of ejector in several regimes is
discussed. Best choice of eddy-viscosity setting is discussed on the
theoretical base. The final verification of the k-ω SST is done on the
base of comparison between experiment and numerical results.
Abstract: The flow field within the combustor of scramjet
engine is very complex and poses a considerable challenge in the
design and development of a supersonic combustor with an optimized
geometry. In this paper comprehensive numerical studies on flow
field characteristics of different cavity based scramjet combustors
with transverse injection of hydrogen have been carried out for both
non-reacting and reacting flows. The numerical studies have been
carried out using a validated 2D unsteady, density based 1st-order
implicit k-omega turbulence model with multi-component finite rate
reacting species. The results show a wide variety of flow features
resulting from the interactions between the injector flows, shock
waves, boundary layers, and cavity flows. We conjectured that an
optimized cavity is a good choice to stabilize the flame in the
hypersonic flow, and it generates a recirculation zone in the scramjet
combustor. We comprehended that the cavity based scramjet
combustors having a bearing on the source of disturbance for the
transverse jet oscillation, fuel/air mixing enhancement, and flameholding
improvement. We concluded that cavity shape with
backward facing step and 45o forward ramp is a good choice to get
higher temperatures at the exit compared to other four models of
scramjet combustors considered in this study.
Abstract: The boundary layer flow and heat transfer on a
stretched surface moving with prescribed skin friction is studied for
permeable surface. The surface temperature is assumed to vary
inversely with the vertical direction x for n = -1. The skin friction at
the surface scales as (x-1/2) at m = 0. The constants m and n are the
indices of the power law velocity and temperature exponent
respectively. Similarity solutions are obtained for the boundary layer
equations subject to power law temperature and velocity variation.
The effect of various governing parameters, such as the buoyancy
parameter λ and the suction/injection parameter fw for air (Pr = 0.72)
are studied. The choice of n and m ensures that the used similarity
solutions are x independent. The results show that, assisting flow (λ >
0) enhancing the heat transfer coefficient along the surface for any
constant value of fw. Furthermore, injection increases the heat
transfer coefficient but suction reduces it at constant λ.
Abstract: Transition prediction of boundary layers has always
been an important problem in fluid mechanics both theoretically and
practically, yet notwithstanding the great effort made by many
investigators, there is no satisfactory answer to this problem. The most
popular method available is so-called e-N method which is heavily
dependent on experiments and experience. The author has proposed
improvements to the e-N method, so to reduce its dependence on
experiments and experience to a certain extent. One of the key
assumptions is that transition would occur whenever the velocity
amplitude of disturbance reaches 1-2% of the free stream velocity.
However, the reliability of this assumption needs to be verified. In this
paper, transition prediction on a flat plate is investigated by using both
the improved e-N method and the parabolized stability equations (PSE)
methods. The results show that the transition locations predicted by
both methods agree reasonably well with each other, under the above
assumption. For the supersonic case, the critical velocity amplitude in
the improved e-N method should be taken as 0.013, whereas in the
subsonic case, it should be 0.018, both are within the range 1-2%.
Abstract: A study of various turbulent inflow generation methods
was performed to compare their relative effectiveness for LES
computations of turbulent boundary layers. This study confirmed
the quality of the turbulent information produced by the family of
recycling and rescaling methods which take information from within
the computational domain. Furthermore, more general inflow methods
also proved applicable to such simulations, with a precursor-like
inflow and a random inflow augmented with forcing planes showing
promising results.